Round-trip GLTF import/export tests

We have many untested steps where bugs can be introduced in GLTF:
- Export from python `gltf_export.py`
- Import in unity `Importer.cs`
- Export from unity `Exporter.cs`
- Import in python `gltf_import.py`

It would be good if we could have several test assets with various GLTF properties (meshes, lighting, transparency, normal maps, etc), and verify that the file is equivalent after passing through each of these steps