static L<Rect> makeGridOfRects(int w, int h, int rw, int rh) {
  new L<Rect> l;
  w = w-mod(w, rw);
  h = h-mod(h, rh);
  for (int y = 0; y < h; y += rh)
    for (int x = 0; x < w; x += rw)
      l.add(Rect(x, y, rw, rh));
  ret l;
}