Blender Git Loki

Fix threading issues of viewport rendering when using movies/generated images

Issue was caused by the fact that guarded allocator is not thread-safe and
generated images/movies could allocate memory when loading pixels to Cycles.

Currently solved by switching memory allocator to using mutex lock (the same
as sued for jobs) when viewport rendering is used.

Nicer solution would be to make guarded allocator thread-safe by using atomic
operations and lock-free lists, but that's more serious change.