Clearcoat isn't energy conservative

[GSterbrant]

Description

The clearcoat shading model does not balance the energy from the base specular. As described here https://github.com/KhronosGroup/glTF/blob/main/extensions/2.0/Khronos/KHR_materials_clearcoat/README.md:

clearcoatFresnel = 0.04 + (1 - 0.04) * (1 - abs(VdotNc))^5
clearcoatAlpha = clearcoatRoughness^2

f_clearcoat = clearcoatFresnel * D(clearcoatAlpha) * G / (4 * abs(VdotNc) * abs(LdotNc))

coated_material = (f_diffuse + f_specular) * (1 - clearcoat * clearcoatFresnel) +
f_clearcoat * clearcoat

We don't apply the 1 - clearcoat * clearcoatFresnel term to balance the energy between the clearcoat and base specular layer.

Furthermore, it looks like clearcoat isn't using specular tint, so we can simplify the Schlick fresnel function for clearcoat.