How to uninstall drag icon set up via drag_source_set_icon()?

[Toralf Lund <toralf procaptura com>]

In one of my Gtkmm applications (using version 2.4.7), I'm setting up a 
"drag source icon" using 
Gtk::Widget::drag_source_set_icon(Glib::RefPtr<Gdk::Colormap> &, 
Glib::RefPtr<Gdk::Pixmap>&, Glib::RefPtr<Gdk::Bitmap>&);

This seems to work fine, but I also want to uninstall the icon some time 
later, and can't figure out how. That is, what I've tried to do, is to 
replace the pixmap/bitmap combination with a stock icon using another 
variant of the above mentioned method. I think this also works in the 
sense that the visible icon changes. However, when looking a bit closer 
at the objects involved, I notice that my pixmap and bitmap will not be 
unreferenced by this operation, i.e. their ref_count remain the same 
after this second drag_source_set_icon(), and the resources do not get 
freed as appropriate.

Is there another way to uninstall or unreference a pixmap/bitmap used as 
drag source icon?

On a somewhat related note, I'm also looking for a nice and simple way 
to render 1-bit data to a Gtk::Pixbuf, as this would be allow me to use 
another variant of the set method - one that's actually recommended by 
the manual... Help, anyone?

- Toralf