static void markPointsInRGBImage(RGBImage img, L<Pt> points) {
  markPointsInRGBImage(img, points, Color.red);
}

static void markPointsInRGBImage(RGBImage img, L<Pt> points, Color color) {
  for (Pt p : points)
    img.setPixel(p.x, p.y, color);
}