anchor.diff:                         (3 :rgb)
                          (4 :rgba))
                        :unsigned-byte
  -                     data)
  +                     (if data data (cffi::null-pointer)))
       (gl:tex-parameter :texture-2d :texture-min-filter (texture-min-filter tex))
       (gl:tex-parameter :texture-2d :texture-mag-filter (texture-mag-filter tex))
       ;; (gl:tex-parameter :texture-2d :texture-min-lod (texture-min-lod tex))
  
positive.code: (defvar *selected-texture-index* nil
    "Current texture in GL context.")
  
  (defun select-texture (tex &key (env-mode :replace))
    "Set TEX as the current gl texture if necessary."
    (if tex
        (progn (unless *selected-texture-index*
                 (gl:enable :texture-2d)
  
negative.code: (defun end-draw ()
    (update-fps)
    (gl:flush))
  
  ;;; Colors helpers
  (defstruct (color (:type (vector float)))
  