scene_graph: Introduce sway_scene_descriptor

Across a wayland compositor, there are multiple shells: It can be
a toplevel, or a layer_shell, or even something more meta like a drag
icon or highlight indicators when dragging windows around.

This object lets us store values that represent these modes of operation
and keep track of what object is being represented.

include/sway/scene_descriptor.h

@@ -0,0 +1,19 @@
+#ifndef _SWAY_SCENE_DESCRIPTOR_H
+#define _SWAY_SCENE_DESCRIPTOR_H
+#include <wlr/types/wlr_scene.h>
+
+enum sway_scene_descriptor_type {
+};
+
+struct sway_scene_descriptor {
+	struct wlr_scene_node *node;
+	enum sway_scene_descriptor_type type;
+	void *data;
+
+	struct wl_listener destroy;
+};
+
+void scene_descriptor_assign(struct wlr_scene_node *node,
+	enum sway_scene_descriptor_type type, void *data);
+
+#endif

sway/meson.build

@@ -8,6 +8,7 @@ sway_sources = files(
 	'lock.c',
 	'main.c',
 	'realtime.c',
+	'scene_descriptor.c',
 	'server.c',
 	'swaynag.c',
 	'xdg_activation_v1.c',

sway/scene_descriptor.c

@@ -0,0 +1,33 @@
+#include <stdlib.h>
+#include "log.h"
+#include "sway/scene_descriptor.h"
+
+static void handle_destroy(struct wl_listener *listener, void *data) {
+	struct sway_scene_descriptor *desc =
+		wl_container_of(listener, desc, destroy);
+
+	desc->node->data = NULL;
+	wl_list_remove(&desc->destroy.link);
+
+	free(desc);
+}
+
+void scene_descriptor_assign(struct wlr_scene_node *node,
+		enum sway_scene_descriptor_type type, void *data) {
+	struct sway_scene_descriptor *desc =
+		calloc(1, sizeof(struct sway_scene_descriptor));
+
+	if (!desc) {
+		sway_log(SWAY_ERROR, "Could not allocate a scene descriptor");
+		return;
+	}
+
+	desc->type = type;
+	desc->data = data;
+	desc->node = node;
+
+	desc->destroy.notify = handle_destroy;
+	wl_signal_add(&node->events.destroy, &desc->destroy);
+
+	node->data = desc;
+}