add surface destroy signal

include/wlr/types/wlr_surface.h

@@ -42,6 +42,7 @@ struct wlr_surface {
 
 	struct {
 		struct wl_signal commit;
+		struct wl_signal destroy;
 	} signals;
 
 	struct wl_list frame_callback_list; // wl_surface.frame

types/wlr_surface.c

@@ -365,6 +365,7 @@ const struct wl_surface_interface surface_interface = {
 
 static void destroy_surface(struct wl_resource *resource) {
 	struct wlr_surface *surface = wl_resource_get_user_data(resource);
+	wl_signal_emit(&surface->signals.destroy, surface);
 
 	wlr_texture_destroy(surface->texture);
 	struct wlr_frame_callback *cb, *next;
@@ -399,6 +400,7 @@ struct wlr_surface *wlr_surface_create(struct wl_resource *res,
 	pixman_region32_init(&surface->pending.opaque);
 	pixman_region32_init(&surface->pending.input);
 	wl_signal_init(&surface->signals.commit);
+	wl_signal_init(&surface->signals.destroy);
 	wl_list_init(&surface->frame_callback_list);
 	wl_resource_set_implementation(res, &surface_interface,
 		surface, destroy_surface);