static L<Rect> makeGridOfRects(int w, int h, int rw, int rh) {
  ret makeGridOfRects(Rect(0, 0, w, h), rw, rh);
}

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