compositor: Ensure views are visible if their output was unplugged

Use the output destroy signal to move the views in the event the output
was unplugged.

Signed-off-by: Zhang, Xiong Y <xiong.y.zhang@intel.com>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
diff --git a/src/compositor.h b/src/compositor.h
index 530de71..84b0f79 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -807,6 +807,7 @@
 	uint32_t output_mask;
 
 	struct wl_listener output_move_listener;
+	struct wl_listener output_destroy_listener;
 };
 
 struct weston_surface {