anchor.diff:               :dimensions dimensions
                :layer-count layer-count
                :samples samples
  -             :viewport (make-viewport dimensions)
  +             :viewport (make-viewport viewport-dims)
                :fixed-sample-locations-p (not (null fixed-sample-locations))))
         ;; return nil as we have nothing that needs to be initialized
         ;; before the fbo can be initialized (no dependancies).
  
positive.code: 
  (defn viewport-for-array ((arr (or null gpu-array))) (or null viewport)
    (when arr
      (make-viewport (gpu-array-dimensions arr) (vec2 0f0 0f0))))
  
  ;;----------------------------------------------------------------------
  
  
negative.code:           (fixed-sample-locations nil))
        (or (first args) '(nil))
      (let ((dimensions (or (listify dimensions)
                            (viewport-dimensions (current-viewport))))
            (viewport-dims (if (= (length dimensions) 1)
                               (list (first dimensions) 1)
                               dimensions)))
        (assert (not (eq (first dimensions) 'quote)) ()
                'quote-symbol-found-in-fbo-dimensions
                :form (first args))
  