[GtkGLExt] Problem going from gtkglext 1.0 to 1.2

[Philipp Klaus Krause <pkk spth de>]

I recently wanted to recompile an application which I haven't touched
for some time. Compiling with gtkglextmm 1.2 gave me some errors, so I
looked at the example code and changed inheritance from public
Gtk::GL::Widget to public Gtk::GL::Widget<SimpleGLScene>. But now I get
compiler errors on gl_* function like set_gl_capability(glconfig):

/usr/local/include/gtkglextmm-1.2/gtkmm/gl/widget.h: In member function
'bool Gtk::GL::Widget<T_GtkWidget>::set_gl_capability(const
Glib::RefPtr<const Gdk::GL::Config>&, bool, int) [with T_GtkWidget =
Display]':
Mainwindow.cc:260:   instantiated from here
/usr/local/include/gtkglextmm-1.2/gtkmm/gl/widget.h:186: error: cannot
convert from base 'Gtk::GL::Widget<Display>' to derived type 'Display'
via virtual base 'Gtk::GL::Widget<Display>'