Blender Git Loki

Fix: BLI_task_test deadlock on windows.

This patch makes BLI_task_scheduler_create wait for all worker threads to have started before
returning to caller. For very short workloads (BLI_taks_test) there is the chance that the
worker threads have not fully started yet, and the main thread is calling pthread_join at
the same time as pthread_setspecific is being called on the worker threads which causes a
deadlock on pthreads4w.

Differential Revision: https://developer.blender.org/D4936

Reviewed By: mont29, sergey, brecht