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.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55901 954f8c5b-7b00-dc11-b283-0030488c597c