drag-action: Capture stage leave

If we are dragging, we don't want the drag handle to emit a
::leave-event even if the pointer left the stage.

clutter/clutter-drag-action.c

@@ -267,6 +267,11 @@ on_captured_event (ClutterActor      *stage,
         emit_drag_end (action, actor, event);
       break;
 
+    case CLUTTER_LEAVE:
+      if (priv->in_drag)
+        return TRUE;
+      break;
+
     default:
       break;
     }