Try to fix a bug where a window move, with a non-rectangular region

Try to fix a bug where a window move, with a non-rectangular region

Try to fix a bug where a window move, with a non-rectangular region unobstructed, would cause
pieces of the window to be moved twice. Effectively, sort of implement a
gdk_draw_drawable_region(drawable, gc, drawable, region, dx, dy);

Am going blind trying to find the off-by-one thing, especially since it doesn't happen except for
the preview tests. Still not fixed, have at it.