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:                                                   (accum-green-size 0)
                                                    (accum-blue-size 0)
                                                    stencil-buffer (stencil-size 0))
    (declare (ignorable accum))
    (without-fp-traps
      (let ((win (make-x11-window :display (glop-x11::x-open-display "")
                                  :screen 0)))
  
negative.code:       (if (and major minor)
            (error "FB Config visual selection not implemented yet.")
            ;; create old style visual
            (let ((attribs (list :rgba ;; no indexed buffer
                                 :red-size red-size
                                 :green-size green-size
                                 :blue-size blue-size
                                 :alpha-size alpha-size
                                 :depth-size depth-size)))
              (when double-buffer
                (push :double-buffer attribs))
              (when stereo
                (push :stereo attribs))
              (when accum-buffer
                (push :accum-red-size attribs)
                (push accum-red-size attribs)
                (push :accum-green-size attribs)
                (push accum-green-size attribs)
                (push :accum-blue-size attribs)
                (push accum-blue-size attribs))
              (setf (x11-window-visual-infos win)
                    (glop-x11::glx-choose-visual (x11-window-display win)
                                                 (x11-window-screen win)
                                                 attribs))))
        ;; create window
        (setf (x11-window-id win) (glop-x11::x-create-window
                                   (x11-window-display win)
  