static BufferedImage renderBackgroundPlusCircles(int w, int h, BackgroundPlus<CircleWithColor> s) {
  BufferedImage bi = newBufferedImage(w, h, toColor(s.background));
  Graphics2D g = imageGraphics(bi);
  for (CircleWithColor e : s.things)
    fillCircle(g, e.center.x, e.center.y, e.r, toColor(e.color));
  g.dispose();
  ret bi;
}