Blender Git Loki

Cycles: use TBB for task pools and task scheduler

No significant performance improvement is expected, but it means we have a
single thread pool throughout Blender. And it should make adding more
parallellization in the future easier.

After previous refactoring commits this is basically a drop-in replacement.
One difference is that the task pool had a mechanism for scheduling tasks to
the front of the queue to minimize memory usage. TBB has a smarter algorithm
to balance depth-first and breadth-first scheduling of tasks and we assume that
removes the need to manually provide hints to the scheduler.