sway: replace noop_output by fallback_output

wlroots removed the support for the noop backend. Instead we rely on the
headless backend to provide the fallback output.

include/sway/tree/root.h

@@ -31,7 +31,7 @@ struct sway_root {
 	list_t *scratchpad; // struct sway_container
 
 	// For when there's no connected outputs
-	struct sway_output *noop_output;
+	struct sway_output *fallback_output;
 
 	struct sway_container *fullscreen_global;