GLTFSerializer New Extensions' Support

[thomlucc]

Update Babylon GLTF Serializer to support more glTF features and extensions.
Some extensions, such Draco and KTX2, will require several steps to being completed.

Milestones:

• EXT_mesh_gpu_instancing
• Draco Compression
  • Draco Encoder (PR)
  • KHR_draco_mesh_compression

[thomlucc]

Thanks for the update @pandaGaume

[myfreeer]

Also consider:

• KHR_mesh_quantization: https://forum.babylonjs.com/t/keep-index-and-khr-mesh-quantization-attributes-on-glb-export/36782
• original image