types/scene: split wlr_scene into modular components

Refactor the scene graph implementation by splitting wlr_scene.c
into multiple focused units: buffer, node, rect and tree. Introduce
corresponding public headers for each component.

This reduces the size and complexity of the core scene file, improves
code organization and makes individual parts of the scene graph easier
to maintain and extend.

As part of this refactor, drop unnecessary pointer indirection when
passing scene trees (e.g. &scene->tree → scene->tree) and update all
call sites accordingly.

No functional changes intended.

examples/cairo-buffer.c

@@ -178,7 +178,7 @@ int main(void) {
 	/* End drawing */
 
 	struct wlr_scene_buffer *scene_buffer = wlr_scene_buffer_create(
-			&server.scene->tree, &buffer->base);
+			server.scene->tree, &buffer->base);
 	if (!scene_buffer) {
 		wl_display_destroy(server.display);
 		return EXIT_FAILURE;