Blender Git Loki

Cycles: Set of fixes for delayed SSS ray tracing

There were multiple issues which are solved now:

- It was possible that ray wouldn't be bounced off the BSSRDF, for example
when PDF or shader eval is zero. In this case PathState might have been
left in pre-bounced state which would have been gave incorrect shading
results.

This is solved by having separate PathState for each of the hits.

- Path radiance summing wasn't happening correct as well, indirect rays
were using wrong path radiance in the case when there were more than
one hit recorded.

This is now using a bit trickier state machine which calculates path
radiance for just SSS (both direct and indirect) and then sums it back
to the final radiance.

- Previous commit wasn't totally correct either and was an induced bug
due to wrong path state left from the "un-happened" ray bounce.

There should be no special case happening here, BSSRDFs will be replaced
with diffuse ones due to PATH_RAY_DIFFUSE_ANCESTOR flag.

- Merged back codebases for "delayed" and "immediate" indirect SSS ray
tracing, hopefully making it easier to maintain the codebase.

Sure this changes brings memory usage back by about 4-5%, but overall
it's still about 2x memory reduction for the experimental kernel here.

Thanks Brecht for the review!