fix bug in LayerTree.map function: replace old value with new value (the result of applying the callback function)

fcore/layer-tree.sml

@@ -66,7 +66,7 @@ struct
           val right = map (f, right)
           val newValue = f value
         in
-          NODE {key = key, value = value, left = left, right = right}
+          NODE {key = key, value = newValue, left = left, right = right}
         end
 
   (* copies non-blank pixels in value vector into acc *)