scene: try to import buffers as textures before rendering

The wlroots APIs currently don't allow importing/uploading a buffer
during rendering operations. Scene-graph buffer nodes need to turn
their wlr_buffer into a wlr_texture at some point. It's not always
possible to do so at wlr_scene_buffer creation time because the
scene-graph may have zero outputs at this point, thus no way to
grab a wlr_renderer.

Instead, add scene-graph buffers to a pending list and try to import
them in wlr_scene_output_commit.

References: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3354

include/wlr/types/wlr_scene.h

@@ -68,6 +68,9 @@ struct wlr_scene {
 	// May be NULL
 	struct wlr_presentation *presentation;
 	struct wl_listener presentation_destroy;
+
+	// List of buffers which need to be imported as textures
+	struct wl_list pending_buffers; // wlr_scene_buffer.pending_link
 };
 
 /** A sub-tree in the scene-graph. */
@@ -114,6 +117,7 @@ struct wlr_scene_buffer {
 	struct wlr_fbox src_box;
 	int dst_width, dst_height;
 	enum wl_output_transform transform;
+	struct wl_list pending_link; // wlr_scene.pending_buffers
 };
 
 /** A viewport for an output in the scene-graph */

types/scene/wlr_scene.c

@@ -130,6 +130,7 @@ void wlr_scene_node_destroy(struct wlr_scene_node *node) {
 		break;
 	case WLR_SCENE_NODE_BUFFER:;
 		struct wlr_scene_buffer *scene_buffer = scene_buffer_from_node(node);
+		wl_list_remove(&scene_buffer->pending_link);
 		wlr_texture_destroy(scene_buffer->texture);
 		wlr_buffer_unlock(scene_buffer->buffer);
 		free(scene_buffer);
@@ -145,6 +146,7 @@ struct wlr_scene *wlr_scene_create(void) {
 	scene_node_init(&scene->node, WLR_SCENE_NODE_ROOT, NULL);
 	wl_list_init(&scene->outputs);
 	wl_list_init(&scene->presentation_destroy.link);
+	wl_list_init(&scene->pending_buffers);
 	return scene;
 }
 
@@ -360,6 +362,9 @@ struct wlr_scene_buffer *wlr_scene_buffer_create(struct wlr_scene_node *parent,
 
 	scene_node_damage_whole(&scene_buffer->node);
 
+	struct wlr_scene *scene = scene_node_get_root(parent);
+	wl_list_insert(&scene->pending_buffers, &scene_buffer->pending_link);
+
 	return scene_buffer;
 }
 
@@ -1113,6 +1118,15 @@ bool wlr_scene_output_commit(struct wlr_scene_output *scene_output) {
 		return true;
 	}
 
+	// Try to import new buffers as textures
+	struct wlr_scene_buffer *scene_buffer, *scene_buffer_tmp;
+	wl_list_for_each_safe(scene_buffer, scene_buffer_tmp,
+			&scene_output->scene->pending_buffers, pending_link) {
+		scene_buffer_get_texture(scene_buffer, renderer);
+		wl_list_remove(&scene_buffer->pending_link);
+		wl_list_init(&scene_buffer->pending_link);
+	}
+
 	wlr_renderer_begin(renderer, output->width, output->height);
 
 	int nrects;