movtextdec: 3GPP TS 26.245 Timed Text Decoder.

This change introduces a basic decoder for 3GPP Timed Text subtitles,
also known as TX3G, Quicktime subtitles, or "movtext" in the existing
code.

This initial change doesn't attempt to parse styling information,
and just reads the plain text of the subtitles. I intend to add
support for styles eventually, but it's challenging due to a lack
of existing players that support them.

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