Extended The Disney BRDF to a BSDF for Mantra renderer

Based on paper: Physically Based Shading at Disney. Brent Burley, Walt Disney Animation Studios

Extended by paper: Extending the Disney BRDF to a BSDF with Integrated Subsurface Scattering. Brent Burley, Walt Disney Animation Studios - 2015

Optimized by implemented Distribution of Visible Normals for BRDF & BTDF: Importance Sampling Microfacet-Based BSDFs with the Distribution of Visible Normals . Eric Heitz and Eugene d’Eon