Move snow the least possible distance.

This diagram shows a driveway. On the left, the white region represents snow on pavement. The colored regions show where the snow could go. The image on the left is the input, the image on the right is the output, mapping the snow from the driveway onto the region where it was moved to indicated by the color.

For each white pixel, find the closest colored region and change the white pixel to that color. There seems to be a lot of neat ways to figure this out. I used rough sampling with expanding circles. (TODO: need to verify this)