Blender Git Loki

BLI_task: Fix/enhance logic of exiting worker threads.

In previous code, worker would exit in case it gets awoken from a condition_wait() and
task queue is empty. However, there may be spurious wake up (either due to pthread itself,
or to some race condition between workers) that would lead to wrongly exiting a worker before
we actually exit the whole scheduler. See code for more details.