anchor.diff:  (defvar *selected-texture-index* nil
     "Current texture in GL context.")
   
  -(defun select-texture (tex &key (env-mode :replace))
  +(defun select-texture (tex &key (env-mode :replace)
  +                                (unit :texture0))
     "Set TEX as the current gl texture if necessary."
  +  (gl:active-texture unit)
     (if tex
         (progn (unless *selected-texture-index*
                  (gl:enable :texture-2d)
  
positive.code:     (gl:vertex 0.0 0.0 scale)))
  
  (defun reshape (width height)
    ;; set viewport to full window
    (gl:viewport 0 0 width height)
    (setf *display-width* width)
    (setf *display-height* height))
  
negative.code:   (gl:matrix-mode :modelview)
    (gl:load-identity)
    (gl:light :light0 :position #(0.0 0.0 0.0 1.0))
    (gl:light :light0 :ambient #(0.2 0.2 0.2 1.0))
    (gl:light :light0 :diffuse #(0.8 0.8 0.8 1.0))
    (gl:light :light0 :specular #(0.5 0.5 0.5 1.0)))
  
  (defun setup-2d-defaults ()
    (gl:clear-color 0.3 0.3 0.3 0)
  