Calculate a "focus overlap" metric

@Wolf Amri suggested having the software show the user a metric of how much in-focus overlap they have in their frames. This could be used for things like:

1. Tell them when they need MORE frames to get a better image

2. Tell them when they have TOO MANY frames, and could set their camera to a bigger differential, or suggest that they remove frames with too much movement, lighting change, etc. and still get a good result.