Update brdf.h

[0x4E69676874466F78]

https://github.com/boksajak/brdf

brdf.h 1.2 - August 2023

Features:

• Added VNDF sampling from "Sampling Visible GGX Normals with Spherical Caps" by Dupuy & Benyoub
• Added Walter's sampling of GG-X distribution
• Added optional reflectance parameter to specify minimal reflectance of dielectrics per material
• Added function specularGGXReflectanceApprox from "Accurate Real-Time Specular Reflections with Radiance Caching" in Ray Tracing Gems by Hirvonen et al.
• All functions and structures put into brdf namespace when compiling as C++
• All functions specified as static when compiling as C++

[w23]

We have our own PBR sampling code at home, an we're not using external brdf.h anymore.