Refactor BSDF into separate models

For sampling/evaluation, the interior is now treated the same as exterior for Dielectric/Metallic BRDF. Specular BSDF takes care of the interface and TIR. This is in line with how the 2015 paper describes the mixing of the 3 models.

Also fixes #78

Update references

Updates

Denoiser is disabled by default
Removed validation screenshots

Fix intructions for building on linux

Russian Roulette, Conditional Compilation

Load anisotropic param from scene file

Fixes for light leaking through backfaces
New scene files

Fixes

1. Fixes for compiling and running on Linux
2. Fixed tiles not rendering correctly when window is resized
3. Fixed issues with instancing