Emission and Clearcoat interaction in gltf_pbr

[bowald]

I was looking at the gltf_pbr shader implementation of emission and it seems to be unaffected by clearcoat. The resulting emission color is plugged into a uniform_edf and then into the surface node.

Based on the spec of the KHR_materials_clearcoat, the clearcoat should preferably be layered on top of the emission. According to the spec it should be darkened by the Fresnel term of the clearcoat.

coated_emission = emission * (1 - clearcoat * clearcoat_fresnel)

where clearcoat_fresnel is Schlick Fresnel using the clearcoat normal and f0 as 0.04