Add new glTF extensions support

[thomlucc]

As new gltF extensions are being ratified, support need to be added in the Exporters:

• KHR_materials_clearcoat
• KHR_materials_sheen
• KHR_materials_transmission
• IOR
• Specular
• Volume
• EXT_mesh_gpu_instancing

[pandaGaume]

@bghgary ClearCoat and Sheen are available into Babylon PBR Material, but how about transmission wich is only covered by alpha (i guess..)?
Also, do we need to create a new dedicated file for each extensions in GLTFEntities project or locate this extensions under the GLTFMaterials , Or GLTFPBRMetallicRoughness ?

[pandaGaume]

@bghgary, @thomlucc, as Max did not support sheen, idea should be to create a Material Plugin, similar to the one used for “games PBR” material such as Metal/Roughness or Specular/Glossiness. Then we introduce our own "Babylon material" to allow new properties and hide others... this should also come with the support of these 2 “games PBR” material. This could be done by script.

[bghgary]

This might be good to check with Autodesk. Let's make sure they are in the loop on how to handle this.

[pandaGaume]

They're on the loop, I had exchange with Brent, and we've already discussed the different options. This proposal is the one he suggests. We may decide to explore this further. We are trying to setting up a call soon to discuss this topic along others.

[bghgary]

Sounds perfect!

[pandaGaume]

As discussed, we need before this, to modify the physical material to beeing exported as PBRMaterial instead as PBRMetallicRoughnessMaterial

[pandaGaume]

The support is coming with

• Milestone 1 : Max2023.
• Milestone 2 : Support of GltfMaterial.
• Milestone 3 : Support additional KHR_Extensions for PhysicalMaterial Max
• Milestone 4 : Support additional KHR_Extensions for PhysicalMaterial Maya