* Box blur: count repeated edge pixels instead of visiting them
The starting window of each horizontal and vertical box pass visited every
position of the radius, including any number outside the image that repeat
the edge pixel, so a radius from a project could make one pass take
arbitrarily long. The pixels inside are summed once and the repeated edge
pixels are added as a count times the edge value.
* address review: skip zero-count edge pixels in the box passes
Dylan M. Taylor
094d59528b