Blender Git Loki

Make switching to threaded malloc safe to be called from threads

For a long time this function was only intended to be used from the main thread,
but since out implementation of parallel range (which is currently only used by
mesh deform modifier) we might want to switch to threaded alloc from object
update thread.

Now we're using spinlock around the check, which makes the code safe to be used
from all over the place.

We might consider using a bit of atomics operations magic there, but it's not so
much important for now, this code is not used in the performance critical code
path.