[Feature]Photon Mapping

I use photon mapping to do the indirect illumination. Indeed, it's way faster than path tracing.Following image is rendered with global photon map and caustic photon map.

[Cornell Box]

Parameters:1000 samples direct lighting
10000 photons
1000 photons for irradiance estimation

Parameters:1000 samples direct lighting
10000 photons
1000 photons for irradiance estimation

Parameters:
2000 samples direct lighting
1500x1500 HDR image as background
10000 photons
1000 photons for irradiance estimation