fix(SceneNode): Parent type should be SceneTree

* Parent type was incorrectly bound to `?*SceneNode` when it should really be `?*SceneTree`. * Closes: https://github.com/swaywm/zig-wlroots/issues/67 * Ref: https://gitlab.freedesktop.org/wlroots/wlroots/-/blob/1712a7d27444d62f8da8eeedf0840b386a810e96/include/wlr/types/wlr_scene.h#L50 * Ref commit: https://github.com/wlrfx/zig-wlroots/commit/e6d90599421e5e024ff4496b9ae79163b6c6e338