evdev: Pass a motion delta to pointer constrain callback

The constrain callback cannot rely on the pointer position of the
corresponding ClutterInputDevice to get the actual delta of the motion
event that is to be constrained since it is only updated when an event is
dispatched. So change the API to pass the previous pointer position when
constraining.

https://bugzilla.gnome.org/show_bug.cgi?id=752752