The idea is to delay shooting indirect rays for the SSS sampling and
trace them after the main integration loop was finished.
This reduces GPU stack usage even further and brings it down to around
652MB (comparing to 722MB before the change and 946MB with previous
stable release).
This also solves the speed regression happened in the previous commit
and now simple SSS scene (SSS suzanne on the floor) renders in 0:50
(comparing to 1:16 with previous commit and 1:03 with official release).
This commit introduces a SSS-oriented intersection structure which is replacing
old logic of having separate arrays for just intersections and shader data and
encapsulates all the data needed for SSS evaluation.
This giver a huge stack memory saving on GPU. In own experiments it gave 25%
memory usage reduction on GTX560Ti (722MB vs. 946MB).
Unfortunately, this gave some performance loss of 20% which only happens on GPU.
This is perhaps due to different memory access pattern. Will be solved in the
future, hopefully.
Famous saying: won in memory - lost in time (which is also valid in other way
around).
The main loop only handles transparent intersections from the camera ray.
Therefore we can simplify some things.
* Avoid PATH_RAY_CAMERA check, this is always true.
* Avoid path_state_next() call, we can just set transparent flag and increase transparent bounces. This way we avoid the function call and some branching.
Also remove debug num_ray_bounces++, this is incorrect here as no indirect bounce happens here.
Should be no functional changes.
Code there started becoming a bit too big, by splitting it up it'll make it
easier to do improvements or extending the features in there.
The layout is not totally final yet, would need to try de-duplicating parts
of code from split kernel with non-split integrators,
