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!

1 files changed, 4 insertions, 2 deletions

diff --git a/intern/cycles/kernel/kernel_types.h b/intern/cycles/kernel/kernel_types.h
index b9869886a8f..017126d05e3 100644
--- a/intern/cycles/kernel/kernel_types.h
+++ b/intern/cycles/kernel/kernel_types.h
@@ -87,7 +87,6 @@ CCL_NAMESPACE_BEGIN
 /* Experimental on GPU */
 #ifdef __KERNEL_EXPERIMENTAL__
 #define __SUBSURFACE__
-#define __SUBSURFACE_DELAYED_INDIRECT__
 #define __CMJ__
 #endif
 
@@ -770,11 +769,14 @@ struct SubsurfaceIntersection
 struct SubsurfaceIndirectRays
 {
 	bool need_update_volume_stack;
-	PathState state;
+	bool tracing;
+	PathState state[BSSRDF_MAX_HITS];
+	PathRadiance direct_L;
 
 	int num_rays;
 	Ray rays[BSSRDF_MAX_HITS];
 	float3 throughputs[BSSRDF_MAX_HITS];
+	PathRadiance L[BSSRDF_MAX_HITS];
 };
 
 /* Constant Kernel Data