While not explicitly stated in the specs, the original author
has stated that S_TEXT/UTF-8 is expected to be text using Subrip
markup, but without Subrip in-band timing.

So, now that we have a decoder that conforms to this expectation,
let's use it.

Note that this change will impact tools that use libavformat. If
they expect srt subtitles to have CODEC_ID_TEXT, they must be
adjusted to expect CODEC_ID_SUBRIP. The actual content is, obviously,
unchanged.
Signed-off-by: Philip Langdale <philipl@overt.org>