Multiple Scattering of fig media in pbrt

Just implemented Jenns and Christensen (1998)'s approach of computing Multiple Scattering of participating media in pbrt.

This is the legal za scene with homogeneous media when only single scattering is accounted. Because there is not much direct lighting, the media is hardly to see.
 

This is the same scene as abve but when both single
And multiple scattering are accounted. Now the presence of media is obvious, thanks to the light Multiple Scattering within the media. This is actually a kind of global illumination for volume effect.

Update: fixed some bugs and tested on a scene with volume caustics.

Single Scattering

With multiple scattering, Volume caustics appeared. Small steps needed to get volume caustics correct.