This commit implements heuristic which allows to skip nodes pushed to the stack
from intersection if distance to them is larger than the distance to the current
intersection.
This should solve speed regression which i didn't notice in the original QBVH
commit (which could have because i had WIP version of this patch applied in my
local branch).
From quick tests speed seems to be much closer to what is was with regular BVH.
There's still some possible code cleanup, but they'll need a bit of assembly
code check and now i want to make it so artists can happily use Cycles over the
holidays.
This commit implements traversal for QBVH tree, which is based on the old loop
code for traversal itself and Embree for node intersection.
This commit also does some changes to the loop inspired by Embree:
- Visibility flags are only checked for primitives.
Doing visibility check for every node cost quite reasonable amount of time
and in most cases those checks are true-positive.
Other idea here would be to do visibility checks for leaf nodes only, but
this would need to be investigated further.
- For minimum hair width we extend all the nodes' bounding boxes.
Again doing curve visibility check is quite costly for each of the nodes and
those checks returns truth for most of the hierarchy anyway.
There are number of possible optimization still, but current state is good
enough in terms it makes rendering faster a little bit after recent watertight
commit.
Currently QBVH is only implemented for CPU with SSE2 support at least. All
other devices would need to be supported later (if that'd make sense from
performance point of view).
The code is enabled for compilation in kernel. but blender wouldn't use it
still.
