[gtkmm] disable clear on expose event

["Jason H." <heblack raft org>]

Can a window's background be turned off so that when there's an expose 
event, the background is unchanged without being cleared?

The below doesn't work here on windows xp:

Glib::RefPtr<Gdk::Window> window = get_window();
window->set_back_pixmap(0,false);

...: no matching function for call to
 `Gdk::Window::set_back_pixmap(int, bool)'
...: candidates are:
  void Gdk::Window::set_back_pixmap(const
  Glib::RefPtr<Gdk::Pixmap>&, bool)

Trying this compiles but causes a crash:

Glib::RefPtr<Gdk::Window> window = get_window();
const Glib::RefPtr<Gdk::Pixmap> *p=0 ;
window->set_back_pixmap(*p,false);