[vtkusers] gtkglext is unnecessary for GtkGLExtVTKRenderWindowInteractor

[Valerie R Coffman]

Hi,

In GtkGLExtRenderWindowInteractor.py, I have replaced references to
gtk.gtkgl.DrawingArea with regular gtk.DrawingArea, and it still works on the
systems I've tested (mac leopard with python 2.5, gtk 2.6.10, and pygtk 2.6.3;
and debian etch with python 2.4, gtk 2.8.20, and pygtk 2.8.6).  Since the
maintenance status of gtkglext is unsteady (see mailing list archives from
August 2007 on http://mail.gnome.org/archives/gtkglext-list/ ), this should be
good news for anyone who wants to use python, gtk, and vtk.

Valerie