[GtkGLExt] gtkglext / gtkglextmm, check for context

[Eric Jonas <jonas mwl mit edu>]

The methods of my Gtk::Gl::DrawingArea that modify the gl state machine
need to be called between: 

Glib::RefPtr<Gdk::GL::Drawable> gldrawable = get_gl_drawable()
if (!gldrawable->gl_begin(get_gl_Context()) 
   return false

call_my_gl_state_modifying_method()

gldrawable->gl_end()

Is there some easy (low overhead) way to check for this from within my
gl-modifying methods, so I don't accidentally call glClear() or whatever
without an active drawable context? 

Thanks, 
			...Eric