Support 3IVD in isom, produced by 3ivx DivX Doctor.

Fixes ticket #486.

libavformat/isom.c

@@ -215,6 +215,8 @@ const AVCodecTag codec_movvideo_tags[] = {
     { CODEC_ID_PRORES, MKTAG('a', 'p', 'c', 'o') }, /* Apple ProRes 422 Proxy */
     { CODEC_ID_PRORES, MKTAG('a', 'p', '4', 'h') }, /* Apple ProRes 4444 */
 
+    { CODEC_ID_MSMPEG4V3, MKTAG('3', 'I', 'V', 'D') }, /* 3ivx DivX Doctor */
+
     { CODEC_ID_NONE, 0 },
 };