Blender Git Loki

Sculpt: OMP -> BLI_task, step I.

This gives usual nice boost in parallelized sections themselves, on global sculpting stroke
it's a bit hard to say, seems like we have an average 5% speedup, but it varies a lot...

Note that this area is only parallelized when you have a big brush over dense geometry, otherwise
there's no change at all.

As a side note, this commit also factorizes code here, three times nearly the same code, tst...