Disable an item from being dragged over a stamped item.

When an item is dragged onto a stamped item, this code moves the dragged item back to its original position and calls "layout" again.  This was tested with jQuery UI but not Draggabilly.
You should be able to use the Rect API. See https://github.com/metafizzy/packery/blob/master/js/rect.js . I think `overlaps` will do it.