Major export improvements

[niello]

Export skin when SkinnedMeshRenderer.bones is empty

Sometimes SkinnedMeshRenderer has an empty array of bones. This is most likely due to an option "Optimize Game Object" on an Import settings Rig tab. This contribution restores skinning information from raw Unity asset data. I can't guarantee that it handles any possible situation but it proved useful in some real cases.

Export skin without animations

Before these changes it was possible to only export both skin & animations or neither of them. Now you can disable animation export but still get skinned meshes exported.

Humanoid animation export

Humanoid animations don't have SRT tracks but instead have special float tracks which control various conceptual properties of the human body. Instead of trying to repeat these calculations, an exporter now plays the final animation in the scene and samples SRT from affected nodes. This allows exporting humanoid animations as regular glTF animations easily.

Export objects without geometry

Commented out a condition which blocked exporting a hierarchy without meshes. It is completely artificial limitation for both Unity and glTF.

Fixes

Accessor bounds (min, max) are now correctly calculated in a case of switching vector handedness.

Minor changes

Empty .bin file is not saved anymore.
Unnecessary fake animation tracks are not allocated.

[niello]

@DavidArayan please consider merging this