Log Message:
Update to 6.8.3.9:

  * GIF comments are no longer displaced by 1 for animated GIFs (reference
    http://www.imagemagick.org/discourse-server/viewtopic.php?f=3&t=22901).
  * A blur kernel with a zero sigma is unity.
  * Do not replace gAMA found in input with 1/2.2
  * Avoid a libpng16 warning about storing unknown chunks.
  * Improve performance when rendering a caption.
  * The compare 'map' image should not be transparent.
  * Use only major version for decorating include path.