Blender Git Loki

Eevee: SSS: Add Christensen-Burley diffusion profile.

This seems to be a correct implementation of the same diffusion profile as Cycles uses by default.

There are a few bias though:
- We consider _A_ the albedo to be 1 when evaluating _s_.
- We use a factor of 0.6 when computing _d_ to match more or less cycles results.

Note that doing per pixel jittering does bias the result even further (loss of energy).