scene: iterate nodes instead of surfaces when rendering

This will allow us to create node types which are rendered but not
surface-based, such as a solid color or image.

include/wlr/types/wlr_scene.h

@@ -67,6 +67,9 @@ struct wlr_scene_surface {
 	struct wl_listener surface_destroy;
 };
 
+typedef void (*wlr_scene_node_iterator_func_t)(struct wlr_scene_node *node,
+	int sx, int sy, void *data);
+
 /**
  * Immediately destroy the scene-graph node.
  */

types/wlr_scene.c

@@ -270,12 +270,20 @@ struct render_data {
 	pixman_region32_t *damage;
 };
 
-static void render_surface_iterator(struct wlr_surface *surface,
+static void render_node_iterator(struct wlr_scene_node *node,
 		int x, int y, void *_data) {
 	struct render_data *data = _data;
 	struct wlr_output *output = data->output;
 	pixman_region32_t *output_damage = data->damage;
 
+	switch (node->type) {
+	case WLR_SCENE_NODE_ROOT:;
+		/* Root node has nothing to render itself */
+		break;
+	case WLR_SCENE_NODE_SURFACE:;
+		struct wlr_scene_surface *scene_surface = scene_surface_from_node(node);
+		struct wlr_surface *surface = scene_surface->surface;
+
 		struct wlr_texture *texture = wlr_surface_get_texture(surface);
 		if (texture == NULL) {
 			return;
@@ -296,6 +304,26 @@ static void render_surface_iterator(struct wlr_surface *surface,
 			output->transform_matrix);
 
 		render_texture(output, output_damage, texture, &box, matrix);
+		break;
+	}
+}
+
+static void scene_node_for_each_node(struct wlr_scene_node *node,
+		int lx, int ly, wlr_scene_node_iterator_func_t user_iterator,
+		void *user_data) {
+	if (!node->state.enabled) {
+		return;
+	}
+
+	lx += node->state.x;
+	ly += node->state.y;
+
+	user_iterator(node, lx, ly, user_data);
+
+	struct wlr_scene_node *child;
+	wl_list_for_each(child, &node->state.children, state.link) {
+		scene_node_for_each_node(child, lx, ly, user_iterator, user_data);
+	}
 }
 
 void wlr_scene_render_output(struct wlr_scene *scene, struct wlr_output *output,
@@ -315,8 +343,8 @@ void wlr_scene_render_output(struct wlr_scene *scene, struct wlr_output *output,
 			.output = output,
 			.damage = damage,
 		};
-		scene_node_for_each_surface(&scene->node, lx, ly,
-			render_surface_iterator, &data);
+		scene_node_for_each_node(&scene->node, lx, ly,
+			render_node_iterator, &data);
 		wlr_renderer_scissor(renderer, NULL);
 	}