shell: Schedule repaint when moving a surface

It seems we used to rely on the repaint scheduled by the cursor motion.
But if there's no cursor, there's no cursor motion, so we need to schedule
our own repaint.

src/shell.c

@@ -782,6 +782,8 @@ move_grab_motion(struct wl_pointer_grab *grab,
 
 	weston_surface_configure(es, dx, dy,
 				 es->geometry.width, es->geometry.height);
+
+	weston_compositor_schedule_repaint(es->compositor);
 }
 
 static void