Blender Git Loki

Movie cache: made it thread safe to operate with memory limitor

Movie cache is using global memory limitor, which isn't thread safe
in some of operations, so it required to add mutex around limitor
operations in movie cache.

It's probably could be solved in a way with less locks involved
bu using different limitor for different areas (like use own limitor
for clips, own limitor for sequencer and so), but that wouldn't be
so easy to control overall memory usage.