[gtkada] Preserving background pixmap when animating

[Emmanuel Briot]

 > Talking about the Gtk_Double_Buffer, why hasn't it made its way in the GtkAda.*
 > hierarchy ? I find it's a very useful widget and use it regularly, so it
 > would be nice to have it "at hand", and not to go and fetch it in the
 > Examples directory.

We do not want to put all the possible widgets in GtkAda itself, so as to keep
the library not too big.
These widgets are more intended for demonstration purposes than for real use.

A similar widget, the GtkAda.Canvas has been put into GtkAda itself, but this
has many many more features and in fact is also much harder to study for
new-comers (although we hope that its use is not too difficult).

Emmanuel