compositor: Introduce weston_output_schedule_repaint()

This lets us schedule a repaint on just the output that needs it.
diff --git a/src/compositor.h b/src/compositor.h
index b437d59..ed09157 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -519,6 +519,8 @@
 void
 weston_output_finish_frame(struct weston_output *output, int msecs);
 void
+weston_output_schedule_repaint(struct weston_output *output);
+void
 weston_output_damage(struct weston_output *output);
 void
 weston_compositor_schedule_repaint(struct weston_compositor *compositor);