srtdec: Add timing-less "subrip" decoder.

After various discussions, we concluded that, amongst other things,
it made sense to have a separate subrip decoder that did not use
in-band timing information, and rather relied on the ffmpeg level
timing.

As this is 90% the same as the existing srt decoder, it's implemented
in the same file.

Signed-off-by: Philip Langdale <philipl@overt.org>

libavcodec/avcodec.h

@@ -441,6 +441,7 @@ enum AVCodecID {
     AV_CODEC_ID_SAMI       = MKBETAG('S','A','M','I'),
     AV_CODEC_ID_REALTEXT   = MKBETAG('R','T','X','T'),
     AV_CODEC_ID_SUBVIEWER  = MKBETAG('S','u','b','V'),
+    AV_CODEC_ID_SUBRIP     = MKBETAG('S','R','i','p'),
 
     /* other specific kind of codecs (generally used for attachments) */
     AV_CODEC_ID_FIRST_UNKNOWN = 0x18000,           ///< A dummy ID pointing at the start of various fake codecs.