Blender Git Loki

Task scheduler: Remove per-pool threads limit

This feature was adding extra complexity to task scheduling
which required yet extra variables to be worried about to be
modified in atomic manner, which resulted in following issues:

- More complex code to maintain, which increases risks of
something going wrong when we modify the code.

- Extra barriers and/or locks during task scheduling, which
causes extra threading overhead.

- Unable to use some other implementation (such as TBB) even for
the comparison tests.

Notes about other changes.

There are two places where we really had to use that limit.

One of them is the single threaded dependency graph. This will
now construct a single-threaded scheduler at evaluation time.
This shouldn't be a problem because it only happens when using
debugging command line arguments and the code simply don't
run in regular Blender operation.

The code seems a bit duplicated here across old and new
depsgraph, but think it's OK since the old depsgraph is already
gone in 2.8 branch and i don't see where else we might want
to use such a single-threaded scheduler.

When/if we'll want to do so, we can move it to a centralized
single-threaded scheduler in threads.c.

OpenGL render was a bit more tricky to port, but basically we
are using conditional variables to wait background thread to
do all the job.