No transparent background for clutter

public class Application : Gtk.Window {

    private GtkClutter.Embed embed;

    public Application () {
        Gtk.Settings.get_default().gtk_application_prefer_dark_theme = true;
        this.window_position = Gtk.WindowPosition.CENTER;
        this.set_default_size (350, 70);

        this.destroy.connect (() => {
            Gtk.main_quit ();
        });

        embed = new GtkClutter.Embed ();

        var stage = embed.get_stage () as Clutter.Stage;
        var color = Clutter.Color().init(255,0,0,100);
        //stage.background_color = color;
        //stage.layout_manager = new Clutter.BinLayout();

        Gtk.IconTheme theme = Gtk.IconTheme.get_default();
        Gdk.Pixbuf? pixbuf = theme.load_icon_for_scale("media-optical-cd-audio", 128, 1, 0);

        Clutter.Image image = new Clutter.Image();
        image.set_data(pixbuf.get_pixels(),
                    pixbuf.has_alpha ? Cogl.PixelFormat.RGBA_8888 : Cogl.PixelFormat.RGB_888,
                    pixbuf.width, pixbuf.height, pixbuf.rowstride);

        var actor = new Clutter.Actor();
        actor.content = image;
        actor.set_size(pixbuf.width, pixbuf.height);

        stage.add_child(actor);
        stage.opacity = 122;

        embed.opacity = 0.5;
        embed.halign = Gtk.Align.START;
        embed.valign = Gtk.Align.START;
        embed.width_request = 100;
        embed.height_request = 100;

        this.add(embed);

        stdout.printf("BG %s\n", stage.background_color_set?"Ja" : "NEIN");
    }

    public static int main (string[] args) {
        Gtk.init (ref args);
        GtkClutter.init (ref args);
        Application app = new Application ();
        app.show_all ();
        Gtk.main ();
        return 0;
    }
}