drag-action: Re-use press_button in emit_drag_end()

Since emit_drag_end() can be called from a MOTION event capture we
cannot call clutter_event_get_button(). We should, instead, use the
press_button value because if we're emitting ::drag-end it means we
also emitted ::drag-begin and the value is valid.

clutter/clutter-drag-action.c

@@ -212,7 +212,7 @@ emit_drag_end (ClutterDragAction *action,
     g_signal_emit (action, drag_signals[DRAG_END], 0,
                    actor,
                    priv->last_motion_x, priv->last_motion_y,
-                   clutter_event_get_button (event),
+                   priv->press_button,
                    clutter_event_get_state (event));
 
   /* disconnect the capture */