[feature] Anisotropic reflections

[Derpius]

Describe the solution you'd like
Replace yocto-gl's isotropic distribution and geometry terms with anisotropic versions.

Additional context
https://jcgt.org/published/0007/04/01/paper.pdf

[Derpius]

Converting roughness and anisotropy parameters to x and y roughness

[Derpius]

After further research it seems there are already established formulas for implementing a mapping from anisotropy+roughness to x and y roughness that are more artist friendly