Add Physically Based Rendering glTF2 extensions: Clear Coat, Transmission and Sheen, etc.

[fire]

Describe the project you are working on

Godot Engine

Describe the problem or limitation you are having in your project

We want to import objects that support a larger range of material from real life.

Describe the feature / enhancement and how it helps to overcome the problem or limitation

This news was mentioned by @akien-mga in developer chat.

Implement Physically Based Rendering glTF2 extensions support for for Clear Coat, Transmission and Sheen.

https://www.khronos.org/news/press/khronos-releases-wave-of-new-gltf-pbr-3d-material-capabilities

• KHR_materials_clearcoat
• KHR_materials_ior
• KHR_materials_sheen
• KHR_materials_specular
• KHR_materials_transmission
• KHR_materials_volume

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Need to determine the work required.

1. Clear coat already exists in Godot Engine.
2. Transmission already exists in Godot Engine.
3. Sheen is not implemented.

If this enhancement will not be used often, can it be worked around with a few lines of script?

Not without writing a gltf2 importer.

Is there a reason why this should be core and not an add-on in the asset library?

Gltf2 is core code.