docs: public static float Mean(ImageSource fastBitmap, int startX, int startY, int width, int height) {
          float mean = 0;
          if (fastBitmap.isGrayscale()) {
              for (int i = startX; i < height; i++) {
                  for (int j = startY; j < width; j++) {
                      mean += fastBitmap.getRGB(j, i);
                  }
              }
              return mean / (width * height);
          } else {
              throw new IllegalArgumentException("ImageStatistics: Only compute mean in grayscale images.");
          }
      }