Blender Git Loki

Camera stabilization fixes and improvements

- Nearest interpolation was always used when there's
no rotation for stabilization. Was a failure of
optimization heuristic.

- Made 2d stabilization frame acquiring threaded.
This function is only used for display and sequencer
which will only benefit of threads here.

- Fixed bug introduced in r48749 which lead to
re-making stable frame on every redraw.