Introduce AVERROR_PATCHWELCOME

Originally committed as revision 10110 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/avcodec.h

@@ -2943,5 +2943,6 @@ int av_parse_video_frame_rate(AVRational *frame_rate, const char *str);
 #define AVERROR_NOFMT       AVERROR(EILSEQ)  /**< unknown format */
 #define AVERROR_NOTSUPP     AVERROR(ENOSYS)  /**< Operation not supported. */
 #define AVERROR_NOENT       AVERROR(ENOENT)  /**< No such file or directory. */
+#define AVERROR_PATCHWELCOME    -MKTAG('P','A','W','E') /**< Not yet implemented in FFmpeg. Patches welcome. */
 
 #endif /* AVCODEC_H */