Blender Git Loki

Fix T51133: Bad performance with texture painting depending on multi-thread settings.

This is more of a temp urgent hack than a proper fix, chenages required
for the later are too involved for 2.80 at that point of time and will
be done later.

That commit merely keeps the whole existing logic for 3D painting
textures, but instead of re-creating a pool of threads for *every* (!)
stroke evaluation (i.e. mouse move event during painting), we instead
use Blender's task scheduler.

That remains sub-optimal, there are likely more improvements possible in
that code, but it should address the main issue reported (which is
presumably caused by windows thread launching being rather heavy process).