OpenGL is now supported in cl-gtk2 with GtkGLExt

Vitaly Mayatskikh sent me some patches for cl-gtk2 to enable GtkGLExt support which I gladly accepted.

GtkGLExt allows attaching OpenGL context to Gtk+ widgets in order to draw on these contexts using OpenGL calls. Lisp already has one binding for OpenGL - the cl-opengl. With the help of cl-gtk2-gtk-glext we can draw with OpenGL inside Gtk+ GUI.