Blender Git Loki

Final fix (I hope!) for partial redraw.

The problem here is that the redraw rectangle can be calculated many
times before actual redraw (for instance due to repetitive calls to the
paint step function for space strokes). Those redraw rectangles need to
be accumulated and finally flushed when the draw call is done.

Also, it is this accumulated rectangle that must account for the
previously drawn rectangle to avoid artifacts due to drawing.

Now all looks like it works correctly :)