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/allcodecs.c

@@ -415,6 +415,7 @@ void avcodec_register_all(void)
     REGISTER_DECODER (REALTEXT, realtext);
     REGISTER_DECODER (SAMI, sami);
     REGISTER_ENCDEC  (SRT, srt);
+    REGISTER_DECODER (SUBRIP, subrip);
     REGISTER_DECODER (SUBVIEWER, subviewer);
     REGISTER_ENCDEC  (XSUB, xsub);