Refactor/split up GltfImport

[atteneder]

The GltfImport class has grown massively over time and currently serves many purposes:

1. Main API entry point (e.g. GltfImport.Load)
2. Orchestration of loading glTFs

• JSON parsing
• Loading Resources from URI (buffers, images)
• Manage asset conversion
  • Mesh Generation
  • Image/Texture Generation
  • Draco/Meshopt decoding
  • Animations

3. Scene instantiation
4. Resource Lifetime Management

• Dispose GltfImport (only) while keeping instances intact #179

That makes this class (and the file it resides in) hard to understand and maintain. We should explore ways in which it could be split up into multiple single-purpose classes/structs.

It could remain the main API entry point, or we use a builder pattern to make setting up a glTF load context easy to read/understand.