evdev: Provide the relative pointer motion event deltas

Compositors need more detailed information about motion events. Make it
possible to retrieve this information when running the evdev backend by
adding the information to the backend specific event struct.

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

clutter/evdev/clutter-evdev.h

@@ -134,6 +134,13 @@ void clutter_evdev_warp_pointer (ClutterInputDevice   *pointer_device,
 CLUTTER_AVAILABLE_IN_1_26
 guint32 clutter_evdev_event_get_event_code (const ClutterEvent *event);
 
+CLUTTER_AVAILABLE_IN_1_26
+gboolean clutter_evdev_event_get_relative_motion (const ClutterEvent *event,
+                                                  double             *dx,
+                                                  double             *dy,
+                                                  double             *dx_unaccel,
+                                                  double             *dy_unaccel);
+
 G_END_DECLS
 
 #endif /* __CLUTTER_EVDEV_H__ */