avformat/riff: add 0x594a TwoCC

sample:

http://samples.mplayerhq.hu/game-formats/wc4-xan/crusader-no-regret.avi

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/riff.c

@@ -411,6 +411,7 @@ const AVCodecTag ff_codec_wav_tags[] = {
     { AV_CODEC_ID_PCM_MULAW,       0x6c75 },
     { AV_CODEC_ID_AAC,             0x706d },
     { AV_CODEC_ID_AAC,             0x4143 },
+    { AV_CODEC_ID_XAN_DPCM,        0x594a },
     { AV_CODEC_ID_G723_1,          0xA100 },
     { AV_CODEC_ID_AAC,             0xA106 },
     { AV_CODEC_ID_SPEEX,           0xA109 },