Blender Git Loki

Blender 2.78c: Fix wrong cycles hair render results when using BVH motion steps

This commit contains all commits required to get proper hair rendering
with BVH motion steps enabled.

The issue here was mainly coming from minimal pixel width feature
which is quite commonly enabled in production shots.

This feature will use some probabilistic heuristic in the curve
intersection function to check whether we need to return intersection
or not. This probability is calculated for every intersection check.
Now, when we use multiple BVH nodes for curve primitives we increase
probability of that primitive to be considered a good intersection
for us. This is similar to increasing minimal width of curve.

What is worst here is that change in the intersection probability
fully depends on exact layout of BVH, meaning probability might
change differently depending on a view angle, the way how builder
binned the primitives and such. This makes it impossible to do
simple check like dividing probability by number of BVH steps.

Other solution might have been to split BVH into fully independent
trees, but that will increase memory usage of all the static
objects in the scenes, which is also not something desirable.

For now used most simple but robust approach: store BVH primitives
time and test it in curve intersection functions. This solves the
regression, but has two downsides:

- Uses more memory.

which isn't surprising, and ANY solution to this problem will
use more memory.

What we still have to do is to avoid this memory increase for
cases when we don't use BVH motion steps.

- Reduces number of maximum available textures on pre-kepler cards.

There is not much we can do here, hardware gets old but we need
to move forward on more modern hardware..