title: OpenCV - Floodfill onto new Mat
body: <p>Given a point on an image, I'd like to floodfill all points connected to that point - but onto a new image.  A naive way to do this would be to floodfill the original image to a special magic colour value.  Then, visit each pixel, and copy all pixels with this magic colour value to the new image.  There must be a better way!</p>