Blender Git Loki

Scheduler fixes: Initialize mutexes correctly and make sure decrementing
the pending_links count in nodes is an atomic operation.

Also moved the child scheduling function into depsgraph eval, where it
is protected by a spin lock to avoid race conditions on
num_pending_links == 0.

NOTE: Apparently there is still a Heisenbug regarding this condition,
but reproducing only works sporadically, have to investigate ...