compositor: Damage output in weston_output_init

So that hotplugged outputs are marked damaged initially.
diff --git a/src/compositor.c b/src/compositor.c
index 8686c1d..a71b7df 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -2314,6 +2314,7 @@
 
 	output->flags = flags;
 	weston_output_move(output, x, y);
+	weston_output_damage(output);
 
 	wl_list_init(&output->frame_callback_list);