compositor: update the surface size when there is a size change without a buffer attach

This fixes at least the case where you want to do wl_viewport.set_destination
to resize the surface but without attaching new content in it.
diff --git a/src/compositor.h b/src/compositor.h
index 057f8be..eae1b20 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -686,6 +686,8 @@
 		 */
 		int32_t width, height;
 	} surface;
+
+	int changed;
 };
 
 struct weston_region {