Production Ready Many Light Sampling

This project will continue development on the Many Light Sampling project from GSoC 2018. It will add support for importance sampling of more features and make that project production ready by stabilizing the existing implementation. The goal of the original Many Light Sampling project was to use multiple importance sampling to avoid having to sample every light source directly for every shading point. The implantation is based on a paper and makes efficient use of a BVH data structure to choose which light source(s) a shading point should sample.