Allow selecting 0x0 rectangles instead of cancelling selection

[pacien]

Example of use case: asking the user to select an area OR a point to capture
either that area or the window in which the point falls in.

This is currently not possible because clicking in the rectangle selection mode
simply cancels the selection altogether, without returning the coordinates of
the click.

Getting a point is possible with -p, but this requires the choice to be made
before calling slurp, which is not desirable in the example use case.