This implements three improvements to the energy preservation and albedo
scaling logic, which help the Principled BSDF pass the white-furnace test
when using the coat layers at high roughness.
Specifically, at roughness 0.3, the albedo scaling brings it from 60% at
the edge to 95%, and with the energy preservation it's 99.8%.
Pull Request: https://projects.blender.org/blender/blender/pulls/134620
9254532b8b