Blender Git Loki

OMP -> BLI_task: BKE's pbvh.c

Should be the last bit of sculpt/paint code, now this is fully using BLI_task.

Note that PBVH normals update is now about 20% quicker than with OMP code
(from 27ms to 21ms with a big stroke over a 500k vertices monkey), even though
a missing thread-protection was added... atomic primitives ftw!

For the records, with the missing `#pragma omp critical` section added,
previous code was like four times slower (above 100ms).