code: public static void toCircle(Bitmap bitmap, boolean antiAliased) {
      Preconditions.checkNotNull(bitmap);
      nativeToCircleFilter(bitmap, antiAliased);
    }
docs: This is a fast, native implementation for rounding a bitmap. It takes the given bitmap and
  modifies it to be circular.
  
  <p>This implementation does not change the underlying bitmap dimensions, it only sets pixels
  that are outside of the circle to a transparent color.
  
  @param bitmap the bitmap to modify