P: Add border detection masks

A use case I have reasonably frequently is a scan of the entire surface of an Instax or Polaroid, including the edges beyond the image. Often I will want to make tweaks to the colors and curves etc of the image, apply a gradient, etc etc. That's all fine as long as the border stays reasonably and uniformly white, but if it takes on a cast, especially a varying cast, that's not so fine.

What I'd like to be able to do is have a mask created for me that includes just the image area, so that I can adjust the image separately from the border area. It seems like this is a common enough use case that there could be an "image area" mask option that detects something that looks like an image surrounded by a relatively uniform color border.

Then I could go to town overlaying gradients, brushes, etc., without having to worry about messing up the nice white border.

Otherwise I have to use brushes and hope auto-mask works, which it doesn't when the image is white AND the border is white (or black/black, or whatever). And if it doesn't then it's zoom into 4x magnification and tiny brush until it looks right, or maybe the healing tool, ....