Multi-importance sampling integration

[pmudry]

Instead of using uniform Monte-Carlo integration for path-tracing, implement Multi-Importance Sampling (MIS). This should greatly reduce noise at low SPP, make everything converge faster.

Useful references :

• https://computergraphics.stackexchange.com/questions/4979/what-is-importance-sampling
• https://lisyarus.github.io/blog/posts/multiple-importance-sampling.html
• https://link.springer.com/chapter/10.1007/978-1-4842-7185-8_20