add dnxhd fourcc

Originally committed as revision 8446 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/isom.c

@@ -120,6 +120,8 @@ const AVCodecTag codec_movvideo_tags[] = {
 
     { CODEC_ID_VC1, MKTAG('v', 'c', '-', '1') }, /* SMPTE RP 2025 */
 
+    { CODEC_ID_DNXHD, MKTAG('A', 'V', 'd', 'n') }, /* AVID DNxHD */
+
     { CODEC_ID_NONE, 0 },
 };