[gtkmm] No spacing for empty pixbufs in a TreeViewColumn

[ERDI Gergo <cactus cactus rulez org>]

Hi,

I'm trying to create a TreeView with two CellRenderers packed into one 
TreeViewColumn. My intention is to make the pixbuf CellRenderer not take 
up any space when the model's data is 0, like in the following example:

            Gtk::TreeModelColumn<Glib::ustring>              col_label;
            Gtk::TreeModelColumn<Glib::RefPtr<Gdk::Pixbuf> > col_pixbuf;
            Glib::RefPtr<Gtk::TreeStore>                     treestore;

    Gtk::TreeView::Column* column = new Gtk::TreeView::Column ("");

    Gtk::CellRendererText* label_renderer = new Gtk::CellRendererText;
    column->pack_start (col_pixbuf, false);
    column->pack_start (*manage (label_renderer), false);

    column->add_attribute(label_renderer->property_text (), col_label);
    label_renderer->property_xalign () = 0;

    append_column(*manage (column));

However, labels are still aligned to the pixbuf if the pixbuf is empty, 
like in the following screenshot:
http://cactus.rulez.org/files/treeview-align.png

So how do I make empty CellRendererPixbufs not take up any space?

Thanks,
	Gergo

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