// alpha is from 0 to 1
static BufferedImage markPointInImageWithAlpha(BufferedImage img, Pt etc p, Color color, double alpha, int size default 3) {
  if (img != null &&p != null)
    rgbFillRect(img, p.x-size/2, p.y-size/2, size, size, colorWithAlpha(color, alpha));
  ret img;
}