Support detecting and demuxing EIA-608 subtitles in mov.

The format is slightly proprietary.
DVDs use a format of
code byte (0x00, 0x01, 0xfe or 0xff), two data bytes
MOV uses instead
cdat/cdt2 atom, two data bytes
Auto-detecting and supporting both in one decoder is trivial,
so a single codec ID is used.

Signed-off-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>

libavcodec/avcodec.h

@@ -414,6 +414,7 @@ enum CodecID {
     CODEC_ID_DVB_TELETEXT,
     CODEC_ID_SRT,
     CODEC_ID_MICRODVD   = MKBETAG('m','D','V','D'),
+    CODEC_ID_EIA_608    = MKBETAG('c','6','0','8'),
 
     /* other specific kind of codecs (generally used for attachments) */
     CODEC_ID_FIRST_UNKNOWN = 0x18000,           ///< A dummy ID pointing at the start of various fake codecs.