Blender Git Loki

OpenGL render: Fix non-deterministic order of frame writes for movies

By the design of task scheduler it was possible that tasks from somewhere
in the middle of scheduled list will be handled first.

For example, one thread might be iterating over the scheduled list and
ignore tasks because there is other thread is working on task from the
same pool. However, if that other thread finishes task before iteration
is over current thread will pick up task from somewhere in in the middle
of the list.

This isn't a problem in general case, but for movie rendering we do need
to have strict order of frames.