Support MP4 subtitles as in

http://samples.mplayerhq.hu/MPEG-4/embedded_subs/1Video_2Audio_2SUBs_timed_text_streams_.mp4

Originally committed as revision 11663 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/isom.c

@@ -171,6 +171,7 @@ const AVCodecTag codec_movaudio_tags[] = {
 
 const AVCodecTag ff_codec_movsubtitle_tags[] = {
     { CODEC_ID_MOV_TEXT, MKTAG('t', 'e', 'x', 't') },
+    { CODEC_ID_MOV_TEXT, MKTAG('t', 'x', '3', 'g') },
     { CODEC_ID_NONE, 0 },
 };