Round-trip GLTF import/export tests

[dylanebert]

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

[thomwolf]

Plus test also the various GLTF extensions