Blender Git Loki

Cycles: equi-angular sampling for homogeneous volumes

This adds an option in the Volume Sampling panel, which helps rendering lamps
inside or near volumes with less noise. It can also increase noise though and
needs improvements to support MIS and heterogeneous volumes, but since it's
useful in some cases already (especially world volumes) it's there now.

Based on the code in the old branch by Stuart, with modifications by Thomas
and Brecht.

Differential Revision: https://developer.blender.org/D291