Blender Git Loki

Cycles: add back random walk subsurface scattering

* shade_surface now samples either a BSDF or BSSRDF to continue the path, and
depending on that will continue with intersect_closest or intersect_subsurface.
* Then intersect_subsurface kernels does ray-tracing and scattering below the
surface. If an exit point is found, it returns to shade_surface.
* Since shade_surface is called twice, direct lighting is evaluated both for
the BSSRDF and any (typically specular) BSDF, since only doing either is
quite noisy.
* Note most files use Christensen-Burley so will render differently.
* subsurface_random_walk should be split up into smaller functions, but I've
kept it like the current implementation now, will split it up in a later commit.