Use correct texture types for PBR Next extensions

[lexaknyazev]

Extension	Slot	Standalone Type
KHR_materials_clearcoat	clearcoatTexture	Red
KHR_materials_clearcoat	clearcoatRoughnessTexture	Red-Green
KHR_materials_transmission	transmissionTexture	Red
KHR_materials_volume	thicknessTexture	Red-Green

Note that the texture type may get promoted when different slots use the same texture. For example:

• Any Red slot combined with Volume gets promoted to Red-Green (e.g. Transmission-Volume or Occlusion-Volume).
• Any Red-Green slot combined with ORM gets promoted to RGB (e.g. ClearcoatRoughness reusing base roughness).