[feature] Energy loss compensation

[Derpius]

Is your feature request related to a problem? Please describe.
Single scattering GGX at high roughness significantly reduces energy, especially at grazing angles, causing a black halo and an overall darkened appearance

Describe the solution you'd like
Yocto-GL provides a ready made compensator for schlick's based conductors based on https://blog.selfshadow.com/publications/s2017-shading-course/imageworks/s2017_pbs_imageworks_slides_v2.pdf, however seems to provide no ready made solution for dielectrics (which makes sense given the drastically increased complexity).

Energy compensation is already used in the conductor, and somewhat integrated into the glossy dielectric BSDF in the form of the Frostbite normalised Disney diffuse, which is pretty much fine given the specular is far less noticeable.
However the specular transmission BSDF is severely affected by the single scattering energy loss, and should compensate for it.

Describe alternatives you've considered
Instead of using energy compensation on the transmission BSDF, we could stochastically sample multiple scattering, given the energy loss is so severe on rough dielectric transmissive surfaces, however it's slow and cumbersome to implement, along with introducing additional variance
https://eheitzresearch.wordpress.com/240-2/

[Derpius]

Example of energy loss in specular transmission on rough surfaces (α = 0.8)

[Derpius]

Added to specular transmission, going to just use schlick's for dielectric (Disney already uses it in the diffuse brdf anyway)