backend: make sure that the resize factor is at least 1

a window may have the stacking rank of 0 (e.g. when it's the only window
on the desktop) and making it the resize factor will lead to incorrect
resizing of the associated regions

src/backend/backend.c

@@ -153,7 +153,7 @@ 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) {
+		if (t && t->stacking_rank > 1) {
 			resize_factor = t->stacking_rank;
 		}
 		resize_region_in_place(&reg_damage, blur_width * resize_factor,