lavf/nut: define fourcc DVBT for DVB_TELETEXT.

Allow "-map 0:s -c:s copy" to copy all subtitles streams.

libavformat/nut.c

@@ -29,6 +29,7 @@ const AVCodecTag ff_nut_subtitle_tags[] = {
     { AV_CODEC_ID_SSA         , MKTAG('S', 'S', 'A',  0 ) },
     { AV_CODEC_ID_DVD_SUBTITLE, MKTAG('D', 'V', 'D', 'S') },
     { AV_CODEC_ID_DVB_SUBTITLE, MKTAG('D', 'V', 'B', 'S') },
+    { AV_CODEC_ID_DVB_TELETEXT, MKTAG('D', 'V', 'B', 'T') },
     { AV_CODEC_ID_NONE        , 0                         }
 };