code: public static String watermark(ThumborUrlBuilder image, int x, int y) {
      if (image == null) {
        throw new IllegalArgumentException("Image must not be null.");
      }
      return watermark(image.toString(), x, y, 0);
    }
docs: This filter adds a watermark to the image.
  
  @param image Watermark image URL. It is very important to understand that the same image
  loader that Thumbor uses will be used here.
  @param x Horizontal position that the watermark will be in. Positive numbers indicate position
  from the left and negative numbers indicate position from the right.
  @param y Vertical position that the watermark will be in. Positive numbers indicate position
  from the top and negative numbers indicate position from the bottom.
  @throws IllegalArgumentException if {@code image} is null.