Crop: Add "boundary" option with a "threshold" option

[bernd-wechner]

I guess, if you really want to conjure, one of the most needed crop features, most missing from tools is this one.

Use case: scanned an image on a flatbed scanner. Want the bounding box crop.

Implementation. Scan the framing pixels if they are uniform within the threshold (however measured) Repeat for each edge, examining the next row/column of pixels in, and while uniform within threshold continue. Stop when threshold exceeded. Thus produced, a bounding box is defined, crop to that.

Ideally, mark it as a rectangle in response to threshold selections.