Refactor bevy_gltf (#15994)

# Objective

Refactor `bevy_gltf`, the criteria for the split is kind of arbitrary
but at least it is not a 2.6k line file.

## Solution

Move methods and structs found in `bevy_gltf/loader.rs` into multiple
new modules.

## Testing

`cargo run -p ci`

crates/bevy_gltf/src/assets.rs

@@ -0,0 +1,315 @@
+//! Representation of assets present in a glTF file
+
+#[cfg(feature = "bevy_animation")]
+use bevy_animation::AnimationClip;
+use bevy_asset::{Asset, Handle};
+use bevy_ecs::{component::Component, reflect::ReflectComponent};
+use bevy_pbr::StandardMaterial;
+use bevy_platform_support::collections::HashMap;
+use bevy_reflect::{prelude::ReflectDefault, Reflect, TypePath};
+use bevy_render::mesh::{skinning::SkinnedMeshInverseBindposes, Mesh};
+use bevy_scene::Scene;
+
+use crate::GltfAssetLabel;
+
+/// Representation of a loaded glTF file.
+#[derive(Asset, Debug, TypePath)]
+pub struct Gltf {
+    /// All scenes loaded from the glTF file.
+    pub scenes: Vec<Handle<Scene>>,
+    /// Named scenes loaded from the glTF file.
+    pub named_scenes: HashMap<Box<str>, Handle<Scene>>,
+    /// All meshes loaded from the glTF file.
+    pub meshes: Vec<Handle<GltfMesh>>,
+    /// Named meshes loaded from the glTF file.
+    pub named_meshes: HashMap<Box<str>, Handle<GltfMesh>>,
+    /// All materials loaded from the glTF file.
+    pub materials: Vec<Handle<StandardMaterial>>,
+    /// Named materials loaded from the glTF file.
+    pub named_materials: HashMap<Box<str>, Handle<StandardMaterial>>,
+    /// All nodes loaded from the glTF file.
+    pub nodes: Vec<Handle<GltfNode>>,
+    /// Named nodes loaded from the glTF file.
+    pub named_nodes: HashMap<Box<str>, Handle<GltfNode>>,
+    /// All skins loaded from the glTF file.
+    pub skins: Vec<Handle<GltfSkin>>,
+    /// Named skins loaded from the glTF file.
+    pub named_skins: HashMap<Box<str>, Handle<GltfSkin>>,
+    /// Default scene to be displayed.
+    pub default_scene: Option<Handle<Scene>>,
+    /// All animations loaded from the glTF file.
+    #[cfg(feature = "bevy_animation")]
+    pub animations: Vec<Handle<AnimationClip>>,
+    /// Named animations loaded from the glTF file.
+    #[cfg(feature = "bevy_animation")]
+    pub named_animations: HashMap<Box<str>, Handle<AnimationClip>>,
+    /// The gltf root of the gltf asset, see <https://docs.rs/gltf/latest/gltf/struct.Gltf.html>. Only has a value when `GltfLoaderSettings::include_source` is true.
+    pub source: Option<gltf::Gltf>,
+}
+
+/// A glTF mesh, which may consist of multiple [`GltfPrimitives`](GltfPrimitive)
+/// and an optional [`GltfExtras`].
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-mesh).
+#[derive(Asset, Debug, Clone, TypePath)]
+pub struct GltfMesh {
+    /// Index of the mesh inside the scene
+    pub index: usize,
+    /// Computed name for a mesh - either a user defined mesh name from gLTF or a generated name from index
+    pub name: String,
+    /// Primitives of the glTF mesh.
+    pub primitives: Vec<GltfPrimitive>,
+    /// Additional data.
+    pub extras: Option<GltfExtras>,
+}
+
+impl GltfMesh {
+    /// Create a mesh extracting name and index from glTF def
+    pub fn new(
+        mesh: &gltf::Mesh,
+        primitives: Vec<GltfPrimitive>,
+        extras: Option<GltfExtras>,
+    ) -> Self {
+        Self {
+            index: mesh.index(),
+            name: if let Some(name) = mesh.name() {
+                name.to_string()
+            } else {
+                format!("GltfMesh{}", mesh.index())
+            },
+            primitives,
+            extras,
+        }
+    }
+
+    /// Subasset label for this mesh within the gLTF parent asset.
+    pub fn asset_label(&self) -> GltfAssetLabel {
+        GltfAssetLabel::Mesh(self.index)
+    }
+}
+
+/// A glTF node with all of its child nodes, its [`GltfMesh`],
+/// [`Transform`](bevy_transform::prelude::Transform), its optional [`GltfSkin`]
+/// and an optional [`GltfExtras`].
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-node).
+#[derive(Asset, Debug, Clone, TypePath)]
+pub struct GltfNode {
+    /// Index of the node inside the scene
+    pub index: usize,
+    /// Computed name for a node - either a user defined node name from gLTF or a generated name from index
+    pub name: String,
+    /// Direct children of the node.
+    pub children: Vec<Handle<GltfNode>>,
+    /// Mesh of the node.
+    pub mesh: Option<Handle<GltfMesh>>,
+    /// Skin of the node.
+    pub skin: Option<Handle<GltfSkin>>,
+    /// Local transform.
+    pub transform: bevy_transform::prelude::Transform,
+    /// Is this node used as an animation root
+    #[cfg(feature = "bevy_animation")]
+    pub is_animation_root: bool,
+    /// Additional data.
+    pub extras: Option<GltfExtras>,
+}
+
+impl GltfNode {
+    /// Create a node extracting name and index from glTF def
+    pub fn new(
+        node: &gltf::Node,
+        children: Vec<Handle<GltfNode>>,
+        mesh: Option<Handle<GltfMesh>>,
+        transform: bevy_transform::prelude::Transform,
+        skin: Option<Handle<GltfSkin>>,
+        extras: Option<GltfExtras>,
+    ) -> Self {
+        Self {
+            index: node.index(),
+            name: if let Some(name) = node.name() {
+                name.to_string()
+            } else {
+                format!("GltfNode{}", node.index())
+            },
+            children,
+            mesh,
+            transform,
+            skin,
+            #[cfg(feature = "bevy_animation")]
+            is_animation_root: false,
+            extras,
+        }
+    }
+
+    /// Create a node with animation root mark
+    #[cfg(feature = "bevy_animation")]
+    pub fn with_animation_root(self, is_animation_root: bool) -> Self {
+        Self {
+            is_animation_root,
+            ..self
+        }
+    }
+
+    /// Subasset label for this node within the gLTF parent asset.
+    pub fn asset_label(&self) -> GltfAssetLabel {
+        GltfAssetLabel::Node(self.index)
+    }
+}
+
+/// Part of a [`GltfMesh`] that consists of a [`Mesh`], an optional [`StandardMaterial`] and [`GltfExtras`].
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-mesh-primitive).
+#[derive(Asset, Debug, Clone, TypePath)]
+pub struct GltfPrimitive {
+    /// Index of the primitive inside the mesh
+    pub index: usize,
+    /// Index of the parent [`GltfMesh`] of this primitive
+    pub parent_mesh_index: usize,
+    /// Computed name for a primitive - either a user defined primitive name from gLTF or a generated name from index
+    pub name: String,
+    /// Topology to be rendered.
+    pub mesh: Handle<Mesh>,
+    /// Material to apply to the `mesh`.
+    pub material: Option<Handle<StandardMaterial>>,
+    /// Additional data.
+    pub extras: Option<GltfExtras>,
+    /// Additional data of the `material`.
+    pub material_extras: Option<GltfExtras>,
+}
+
+impl GltfPrimitive {
+    /// Create a primitive extracting name and index from glTF def
+    pub fn new(
+        gltf_mesh: &gltf::Mesh,
+        gltf_primitive: &gltf::Primitive,
+        mesh: Handle<Mesh>,
+        material: Option<Handle<StandardMaterial>>,
+        extras: Option<GltfExtras>,
+        material_extras: Option<GltfExtras>,
+    ) -> Self {
+        GltfPrimitive {
+            index: gltf_primitive.index(),
+            parent_mesh_index: gltf_mesh.index(),
+            name: {
+                let mesh_name = gltf_mesh.name().unwrap_or("Mesh");
+                if gltf_mesh.primitives().len() > 1 {
+                    format!("{}.{}", mesh_name, gltf_primitive.index())
+                } else {
+                    mesh_name.to_string()
+                }
+            },
+            mesh,
+            material,
+            extras,
+            material_extras,
+        }
+    }
+
+    /// Subasset label for this primitive within its parent [`GltfMesh`] within the gLTF parent asset.
+    pub fn asset_label(&self) -> GltfAssetLabel {
+        GltfAssetLabel::Primitive {
+            mesh: self.parent_mesh_index,
+            primitive: self.index,
+        }
+    }
+}
+
+/// A glTF skin with all of its joint nodes, [`SkinnedMeshInversiveBindposes`](bevy_render::mesh::skinning::SkinnedMeshInverseBindposes)
+/// and an optional [`GltfExtras`].
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-skin).
+#[derive(Asset, Debug, Clone, TypePath)]
+pub struct GltfSkin {
+    /// Index of the skin inside the scene
+    pub index: usize,
+    /// Computed name for a skin - either a user defined skin name from gLTF or a generated name from index
+    pub name: String,
+    /// All the nodes that form this skin.
+    pub joints: Vec<Handle<GltfNode>>,
+    /// Inverse-bind matrices of this skin.
+    pub inverse_bind_matrices: Handle<SkinnedMeshInverseBindposes>,
+    /// Additional data.
+    pub extras: Option<GltfExtras>,
+}
+
+impl GltfSkin {
+    /// Create a skin extracting name and index from glTF def
+    pub fn new(
+        skin: &gltf::Skin,
+        joints: Vec<Handle<GltfNode>>,
+        inverse_bind_matrices: Handle<SkinnedMeshInverseBindposes>,
+        extras: Option<GltfExtras>,
+    ) -> Self {
+        Self {
+            index: skin.index(),
+            name: if let Some(name) = skin.name() {
+                name.to_string()
+            } else {
+                format!("GltfSkin{}", skin.index())
+            },
+            joints,
+            inverse_bind_matrices,
+            extras,
+        }
+    }
+
+    /// Subasset label for this skin within the gLTF parent asset.
+    pub fn asset_label(&self) -> GltfAssetLabel {
+        GltfAssetLabel::Skin(self.index)
+    }
+}
+
+/// Additional untyped data that can be present on most glTF types at the primitive level.
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-extras).
+#[derive(Clone, Debug, Reflect, Default, Component)]
+#[reflect(Component, Default, Debug)]
+pub struct GltfExtras {
+    /// Content of the extra data.
+    pub value: String,
+}
+
+impl From<&serde_json::value::RawValue> for GltfExtras {
+    fn from(value: &serde_json::value::RawValue) -> Self {
+        GltfExtras {
+            value: value.get().to_string(),
+        }
+    }
+}
+
+/// Additional untyped data that can be present on most glTF types at the scene level.
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-extras).
+#[derive(Clone, Debug, Reflect, Default, Component)]
+#[reflect(Component, Default, Debug)]
+pub struct GltfSceneExtras {
+    /// Content of the extra data.
+    pub value: String,
+}
+
+/// Additional untyped data that can be present on most glTF types at the mesh level.
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-extras).
+#[derive(Clone, Debug, Reflect, Default, Component)]
+#[reflect(Component, Default, Debug)]
+pub struct GltfMeshExtras {
+    /// Content of the extra data.
+    pub value: String,
+}
+
+/// Additional untyped data that can be present on most glTF types at the material level.
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-extras).
+#[derive(Clone, Debug, Reflect, Default, Component)]
+#[reflect(Component, Default, Debug)]
+pub struct GltfMaterialExtras {
+    /// Content of the extra data.
+    pub value: String,
+}
+
+/// The material name of a glTF primitive.
+///
+/// See [the relevant glTF specification section](https://registry.khronos.org/glTF/specs/2.0/glTF-2.0.html#reference-material).
+#[derive(Clone, Debug, Reflect, Default, Component)]
+#[reflect(Component)]
+pub struct GltfMaterialName(pub String);