Similar to slurp's -p flag, but with more features.
In slurp, -p will ignore predefined rectangles. We don't support those, but we support selecting windows with a single click. I think our equivalent should return:

• the ID of the window that was clicked
• its geometry
• the coordinates of the point that was clicked (when selecting a rectangle, it might also be interesting to return the point where the button was released here)