LTC Area Light

TODO:

http://patapom.com/blog/BRDF/MSBRDFEnergyCompensation2/

Reference:

Real-Time Polygonal-Light Shading with Linearly Transformed Cosines. Eric Heitz, Jonathan Dupuy, Stephen Hill and David Neubelt. ACM Transactions on Graphics (Proceedings of ACM SIGGRAPH 2016) 35(4), 2016. Project page: https://eheitzresearch.wordpress.com/415-2/