Implement micro-facet roughness (anisotropic/isotropic)

[adriengivry]

Problem this feature should fix

Roughness at a microscopic scale can have a vast impact on how light is scattered, Overload's roughness approximation doesn't apply micro-facet theory.

Expected solution

Documentation on the desired effect:
https://pbr-book.org/4ed/Reflection_Models/Roughness_Using_Microfacet_Theory

This could be helpful for surfaces like hair and brushed metal. Could be added to the standard shader as a feature