gltfpack: Implement separate animation parsing
gltfpack: Implement separate animation parsing

We now have a struct Animation and struct Track for animation data, and
extract this information from .gltf files; this will allow us to expand
the animation compression / analysis to be similar to that for meshes.

For now we still use the old animation path to actually export
animations.

zeux/meshoptimizer,zeux/meshoptimizer,zeux/meshoptimizer