[gtkada] run time error only on linux

[Arnaud Charlet]

> I don't know if it would be appreciated, but I think I have found a bug 
> on linux implementation (which does not occurs on windows...).
> to demonstrate that, I have made a simple example wich was originally 
> created by Glade, which compile successfully, but a runtime error occurs 
> on code line :
> gdk.image.get(main_window.image,w,0,0,width,height) file window_pkg.adb, 
> line 27

It's always helpful to explain exactly what you saw (do not assume that
this can be reproduced easily), and why you think this is a bug in Gtk or
GtkAda, as opposed to e.g a bug in your application, thanks.

Arno