GtkGLExt is an OpenGL extension to GTK 2.0 or later.

  GtkGLExt provides the GDK objects to support OpenGL rendering in GTK,
and GtkWidget API add-ons to make GTK+ widgets OpenGL-capable.  As opposed
to Jane Loff's GtkGLArea, it does not provide any OpenGL widget, but an
interface to use OpenGL on *ANY* GTK+ widget.