[feature] Bidirectional path tracing

[Derpius]

Is your feature request related to a problem? Please describe.
While diffuse lighting already has barely any noise, indirect diffuse lighting and more importantly caustics are poor, and in the case of caustics they’re not effected by direct light.

Describe the solution you'd like
BDPT is an unbiased method for drastically reducing the variance created by these light interactions, compared to the arguably simpler to implement but biased photon mapping.

Unlike with backwards path tracing, there’s no simple step by step guides that explain the logic (e.g. scratchapixel), however after reading a paper and a few articles on it, it doesn’t seem entirely out of my reach, and wouldn’t take "much" more to upgrade to metropolis light transport.

[Derpius]

Started work on this now, oh boy.