ComplexLit Shader

Add clearcoat support in comparsion with Lit shader.

Material information store in KHR_materials_clearcoatExtension.

clearcoatFactor & clearcoatRoughnessFactor is ready to use.

clearcoatTexture is not fully compatible with official implementation, but still works fine on Unity with this SDK.

The Clearcoat property is an additional specular layer that is applied on top of a material. An example of this in effect is when a car is painted, and the paint acts as the additional layer.