add FFDS fourcc, ok'd by kostya

fixes FFDS files in http://tranquillity.ath.cx/uncommon_video_codecs_final.txt

Originally committed as revision 15989 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/riff.c

@@ -53,6 +53,7 @@ const AVCodecTag codec_bmp_tags[] = {
     { CODEC_ID_MPEG4,        MKTAG('S', 'E', 'D', 'G') },
     { CODEC_ID_MPEG4,        MKTAG('R', 'M', 'P', '4') },
     { CODEC_ID_MPEG4,        MKTAG('3', 'I', 'V', '2') },
+    { CODEC_ID_MPEG4,        MKTAG('F', 'F', 'D', 'S') },
     { CODEC_ID_MSMPEG4V3,    MKTAG('D', 'I', 'V', '3') }, /* default signature when using MSMPEG4 */
     { CODEC_ID_MSMPEG4V3,    MKTAG('M', 'P', '4', '3') },
     { CODEC_ID_MSMPEG4V3,    MKTAG('M', 'P', 'G', '3') },