riff: Add mpgv MPEG-2 fourcc

Supported by mplayer and seen in the wild.
(cherry picked from commit 505345ed5d)

libavformat/riff.c

@@ -130,6 +130,7 @@ const AVCodecTag ff_codec_bmp_tags[] = {
     { CODEC_ID_MPEG2VIDEO,   MKTAG('s', 'l', 'i', 'f') },
     { CODEC_ID_MPEG2VIDEO,   MKTAG('E', 'M', '2', 'V') },
     { CODEC_ID_MPEG2VIDEO,   MKTAG('M', '7', '0', '1') }, /* Matrox MPEG2 intra-only */
+    { CODEC_ID_MPEG2VIDEO,   MKTAG('m', 'p', 'g', 'v') },
     { CODEC_ID_MJPEG,        MKTAG('M', 'J', 'P', 'G') },
     { CODEC_ID_MJPEG,        MKTAG('L', 'J', 'P', 'G') },
     { CODEC_ID_MJPEG,        MKTAG('d', 'm', 'b', '1') },