DM-1128

New PR for code review of DM-1128, span-based grow operations for Footprint
I considered the case of asymmetric structuring elements: they aren't necessary for this use case, but they'd enable us to use the same code to implement the grow up/down/left/right functionality, which would probably be a win. Given your comment, I'll take another look and see if I can convince myself it's worth doing something smarter here.

Thanks for the review!