anchor.diff:  (defcfun ("glXChooseVisual" %glx-choose-visual) visual-info
     (display-ptr :pointer) (screen :int) (attribs :pointer))
   
  -(defun glx-choose-visual (dpy screen &rest attribs)
  +(defun glx-choose-visual (dpy screen attribs)
     (with-foreign-object (atts :int (1+ (length attribs)))
       (loop for i below (length attribs)
            for attr = (nth i attribs)
  
positive.code:       (if (and major minor)
            (error "FB Config visual selection not implemented yet.")
            ;; create old style visual
            (setf (x11-window-visual-infos win)
                  (glop-x11::glx-choose-visual (x11-window-display win)
                                               (x11-window-screen win)
                                               :rgba
                                               :red-size 4
                                               :green-size 4
                                               :blue-size 4
                                               :alpha-size (if alpha-size 4 0)
                                               :depth-size depth-size
                                               (if double-buffer :double-buffer :single-buffer))))
        ;; create window
        (setf (x11-window-id win) (glop-x11::x-create-window
                                   (x11-window-display win)
  
negative.code:                                                   (accum-green-size 0)
                                                    (accum-blue-size 0)
                                                    stencil-buffer (stencil-size 0))
    (without-fp-traps
      (let ((win (make-x11-window :display (glop-x11::x-open-display "")
                                  :screen 0)))
  