output: Destroy when output layout is destroyed

Since output layout is destroyed when the wayland display is destroyed
we run into a destroy listener order problem: Either the display starts
destroying the outputs first, in which case we're good: The existing
handling will clean up. However, things go wrong if the display decides
to destroy the output layout first. In this case, sway will hold
invalid references to the output layout as part of each output so that
when it finally goes to destroy them, sway will dereference destroyed
output layout bits.

Ref: #6844 (comment)

include/sway/output.h

@@ -39,6 +39,7 @@ struct sway_output {
 
 	struct sway_output_state current;
 
+	struct wl_listener layout_destroy;
 	struct wl_listener destroy;
 	struct wl_listener commit;
 	struct wl_listener present;

sway/desktop/output.c

@@ -892,8 +892,7 @@ static void update_output_manager_config(struct sway_server *server) {
 	ipc_event_output();
 }
 
-static void handle_destroy(struct wl_listener *listener, void *data) {
-	struct sway_output *output = wl_container_of(listener, output, destroy);
+static void begin_destroy(struct sway_output *output) {
 	struct sway_server *server = output->server;
 
 	if (output->enabled) {
@@ -904,6 +903,7 @@ static void handle_destroy(struct wl_listener *listener, void *data) {
 
 	wl_list_remove(&output->link);
 
+	wl_list_remove(&output->layout_destroy.link);
 	wl_list_remove(&output->destroy.link);
 	wl_list_remove(&output->commit.link);
 	wl_list_remove(&output->present.link);
@@ -922,6 +922,16 @@ static void handle_destroy(struct wl_listener *listener, void *data) {
 	update_output_manager_config(server);
 }
 
+static void handle_destroy(struct wl_listener *listener, void *data) {
+	struct sway_output *output = wl_container_of(listener, output, destroy);
+	begin_destroy(output);
+}
+
+static void handle_layout_destroy(struct wl_listener *listener, void *data) {
+	struct sway_output *output = wl_container_of(listener, output, layout_destroy);
+	begin_destroy(output);
+}
+
 static void update_textures(struct sway_container *con, void *data) {
 	container_update_title_textures(con);
 	container_update_marks_textures(con);
@@ -1036,6 +1046,8 @@ void handle_new_output(struct wl_listener *listener, void *data) {
 	output->server = server;
 	wlr_damage_ring_init(&output->damage_ring);
 
+	wl_signal_add(&root->output_layout->events.destroy, &output->layout_destroy);
+	output->layout_destroy.notify = handle_layout_destroy;
 	wl_signal_add(&wlr_output->events.destroy, &output->destroy);
 	output->destroy.notify = handle_destroy;
 	wl_signal_add(&wlr_output->events.commit, &output->commit);