Naive caustics rendering via light tracing

Light tracing is implemented, where photons (infinitesimal) are traced and splat onto the canvas for better caustics rendering in scenes with multi-layer specular materials. This can be further improved, by splatting the anisotropic photons (like, Gaussians). The implementation is not profiled, and it is a naive algorithm currently.

No caustics	Blend light tracing with path tracing	Caustics