Fix shape modification not updating graphics in testbed

[agarret7]

This PR is for a fix for the GraphicsManager::remove_collider_nodes function in the testbed. The problem was that the entity was despawned but not removed from GraphicsManager::b2sn, creating an invalid state. See this thread for the discussion.

TestbedApp::add_callback still requires a manual refresh with gfx.add_collider if the user wants to see the 3D entity update correctly.

if let Some(gfx) = &mut gfx {
    gfx.remove_collider(ball_coll_handle, &physics.colliders);
    gfx.add_collider(ball_coll_handle, &physics.colliders);
}

debug_shape_modification3 before:

debug_shape_modification3 after:

[Vrixyz]

Suggested change

	if let Some(sns) = self.b2sn.remove(&body) {
	let mut new_sns = vec![];
	for sn in sns.into_iter() {
	if let Some(sn_c) = sn.collider {
	if sn_c == collider {
	commands.entity(sn.entity).despawn();
	} else {
	new_sns.push(sn);
	}
	}
	}
	self.b2sn.insert(body, new_sns);
	if let Some(mut sns) = self.b2sn.remove(&body) {
	sns = sns
	.into_iter()
	.filter(|sn| {
	let Some(sn_c) = sn.collider else {
	return false;
	};
	if sn_c == collider {
	commands.entity(sn.entity).despawn();
	return false;
	} else {
	return true;
	}
	})
	.collect();
	self.b2sn.insert(body, sns);
	}

Small nitpick, I did that just to help me understand the change, not sure if which is better, don´t worry about it I'll merge one or the other after discussion with seb.

[Vrixyz]

For the record, I think the best solution is to monitor changes over the shapes, and then modify the mesh handle:

In practice, running these snippets of code:

• recompute the mesh:

rapier/src_testbed/objects/node.rs

Lines 82 to 85 in eba44e7

	let mesh = prefab_meshs
	.get(&shape.shape_type())
	.cloned()
	.or_else(|| generate_collider_mesh(shape).map(|m| meshes.add(m)));

• Then reapply the mesh handle on the entity.

But as we're in testbed, I think this solution is good enough :)

[agarret7]

I had thought that it might be good to factor these sorts of updates into the testbed loop, but wasn't sure how to track incremental computation of the state. Especially since I noticed you have a vector of flags already tracking collider updates in ColliderSet::changes private to the rapier3d crate and that collider.set_shape uses this internally:

rapier/src/geometry/collider.rs

Lines 445 to 449 in eba44e7

	/// Sets the shape of this collider.
	pub fn set_shape(&mut self, shape: SharedShape) {
	self.changes.insert(ColliderChanges::SHAPE);
	self.shape = shape;
	}

It seems like monitoring those changes from the testbed requires a new function like pub fn get_changes() -> &ColliderChanges? From what I could tell the ColliderChanges otherwise cannot be seen outside the main crate and thus wasn't sure how you might select meshes to be recomputed.