Add "DAVC" FourCC

This FourCC is used by "mpegable AVC" codec and the file encoded with this codec decodes correctly with FFmpeg's H264 decoder.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/riff.c

@@ -33,6 +33,7 @@ const AVCodecTag ff_codec_bmp_tags[] = {
     { CODEC_ID_H264,         MKTAG('X', '2', '6', '4') },
     { CODEC_ID_H264,         MKTAG('x', '2', '6', '4') },
     { CODEC_ID_H264,         MKTAG('a', 'v', 'c', '1') },
+    { CODEC_ID_H264,         MKTAG('D', 'A', 'V', 'C') },
     { CODEC_ID_H264,         MKTAG('V', 'S', 'S', 'H') },
     { CODEC_ID_H263,         MKTAG('H', '2', '6', '3') },
     { CODEC_ID_H263,         MKTAG('X', '2', '6', '3') },