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

clutter/evdev/clutter-evdev.h

@@ -82,6 +82,8 @@ void  clutter_evdev_reclaim_devices (void);
  */
 typedef void (*ClutterPointerConstrainCallback) (ClutterInputDevice *device,
 						 guint32             time,
+						 float               prev_x,
+						 float               prev_y,
 						 float              *x,
 						 float              *y,
 						 gpointer            user_data);