Add new extensions
Adds support for the following extensions:
- KHR_materials_clearcoat
- KHR_materials_volume
- KHR_materials_iridescence
- KHR_materials_transmission
- KHR_materials_sheen
- KHR_materials_specular
- EXT_mesh_gpu_instancing
... and fixes minor issues in extensions:
- KHR_texture_transform
- KHR_materials_pbrSpecularGlossiness
Note that this change also adds the string constant for the KHR_materials_variants
extension, but it does not implement it. This will be done later if needed.