Allow decoders to set AVCodecContext.refs.

Originally committed as revision 16305 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -2082,7 +2082,7 @@ typedef struct AVCodecContext {
     /**
      * number of reference frames
      * - encoding: Set by user.
-     * - decoding: unused
+     * - decoding: Set by lavc.
      */
     int refs;