Add flag to say whether to crop or pad

[Pomax]

image comparison only really works when the images are the same size, and there are two ways to effect that: cropping the larger image to the smaller image's size, or padding the smaller image to the larger image's size.

In both cases, we probably want to use something that can evaluate alginment really quickly so that instead of cropping blindly (e.g. padding or chopping off the right and/or bottom), we can "center" the smaller image wrt the larger image, and then pad/crop in a way that makes sure that diffing flags as few "moved" diffs as possible.