Blender Git Loki

Make 3d projpaint iteration lockless.

While speedup is hard to detect (highly fluctuent), it seems to be
around 5% on average on my 8 threads machine...

It also remove usage of a 'global' thread lock, which is always good.

Note that I also tried to use proper foreach threaded iterator construct
(see D5372), but that proved to be relatively slower (presumably due to
the very high dissymmetry between tasks, usually during a paint stroke
only a few chunks will require most of the computing effort, overhead of
threaded foreach management is then noticeable).

This concludes (for now) the work on
T51133 Bad performance with texture painting depending on multi-thread settings.