Blender Git Loki

Use generic task scheduler for threaded image processor

It allows to schedule tasks of smaller size without having
threads overhead or extra worry about splitting tasks into
smaller pieces.

This simplifies code in color management which was manually
splitting task into smaller chunks to keep memory usage low.

Further optimization is possible by avoid malloc called from
threads, but that's how it used to work for ages already
and would be optimized as a separate patch.