Several changes

Still alot to add and cleanup, but this at least makes sure fbx-conv and new3dapi are in sync.

This splits the reader into two parts: FbxMeshInfo and FbxConverter. It also uses another strategy: where the original version exported everything by iterating over the nodes and fetching the info, this version fetches and caches the info up front, resulting in smaller meshes, better reusing instances etc.

- Add packing colors
- Add texture usage
- Add uv mapping