// m = cols, n = rows
static L<Rect> tileRects(int w, int h, int m, int n) {
  new L<Rect> tiles;
  for y to n: {
    int y1 = y*h/n, y2 = (y+1)*h/n;
    for x to m: {
      int x1 = x*w/m, x2 = (x+1)*w/m;
      Rect r = rectFromPoints(x1, y1, x2, y2);
      tiles.add(r);
    }
  }
  ret tiles;
}