* gtk/Gtk.metadata: hide PrintContext.get_CairoContext.
* gtk/PrintContext.custom: manual get_cairo_context implementation.
Mono.Cairo assumes it is wrapping owned references, so we need to
take a ref out on the returned cairo_t pointer.
* sample/GtkDemo/DemoPrinting.cs: dispose the CairoContext in
to be a good citizen and avoid warnings.
svn path=/trunk/gtk-sharp/; revision=100942
