* gltfio: add Asset/Resource extended implementations
- Add gltfio/src/extended to implement an alternate loader for
primitives. This is largely based on the implementation in
AssetLoader/ResourceLoader
- Able to correctly produce flat shading from gltf that only have
vertex positions and indices.
- This is not hooked into current code and should have no
practical effect on gltfio.
c8098a4c15