Blender Git Loki

Optimizations for image paint undo stack for drag dot/anchored strokes

* Skip copy of undo tile to temorary buffer when restoring. Gives a nice
speedup.
* When we restore the image from undo tiles, make tiles invalid. On undo
push end, invalid tiles get deleted. This solves saving too many tiles
in the case where user could happilly drag the dot all around the image.