This attempts to port the GTK example away from
the GTK 2-era GtkGLArea library and PyGTK
to modern PyGObject and GTK’s built-in GtkGLArea.

Also switched to vtkGenericOpenGLRenderWindow since
that should work with custom contexts.

Gtk.Widget.get_pointer method is also deprecated so I switched
used GtkEventControllerMotion.

Currently does not work	– displays garbled mess in the widget.

vtk/vtk#18395