backend: fix resize factor calculation

the corresponding regions need to be resized once for each window in the
stack above the damaged window including the damaged window itself. we
were off by one.

src/backend/backend.c

@@ -154,7 +154,7 @@ void paint_all_new(session_t *ps, struct managed_win *t) {
 		// TODO(yshui): maybe we don't need to resize reg_damage, only reg_paint?
 		int resize_factor = 1;
 		if (t) {
-			resize_factor = t->stacking_rank;
+			resize_factor = t->stacking_rank + 1;
 		}
 		resize_region_in_place(&reg_damage, blur_width * resize_factor,
 		                       blur_height * resize_factor);