add new tag 'CAVS' for Chinese AVS codec

Originally committed as revision 5847 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/riff.c

@@ -151,6 +151,7 @@ const CodecTag codec_bmp_tags[] = {
     { CODEC_ID_CSCD, MKTAG('C', 'S', 'C', 'D') },
     { CODEC_ID_ZMBV, MKTAG('Z', 'M', 'B', 'V') },
     { CODEC_ID_KMVC, MKTAG('K', 'M', 'V', 'C') },
+    { CODEC_ID_CAVS, MKTAG('C', 'A', 'V', 'S') },
     { CODEC_ID_RAWVIDEO, 0 },
     { CODEC_ID_NONE, 0 },
 };