BRDF Refactoring

[acavalier]

Talking with @trethaller about a future BRDF refactoring in Heaps and Hide:

• Need to fix the typo: BRDF (instead of BDRF)
• Move BRDFs and utility functions (fresnel, isotropic/anisotropic GGX, Smith Masking Term, sampling, roughness remapping, etc.) in one file to ensure a coherent shading within the engine and the editor (maybe on Heaps side and report changes in Hide accordingly)