Feature Request: Screenshot background windows

Also mentioned here: https://github.com/go-vgo/robotgo/issues/561 but in a list of others, so I thought I'd create an issue specifically for this feature request.

The ability to capture only the contents of a background window would be extremely useful for those who want to perform some sort of background processing (ie with OpenCV) based on a 'screenshot' of a window that isn't the 'active' one.