Respect channel expansion settings when creating the initial mediatype
LAV Splitter: Fix the filtering of non-forced PGS subtitles.

PGS segments with just a header were mistakenly ignored. This included "end of display" segments so it could result in no subtitle being displayed at all.
DVD Subtitles: Really obey the DVD Composit event or the forced sub flag.
DVD Subtitles: Try to improve the handling of subtitles without proper timings.

Ensure the stop time is always checked and don't change the timings when they are unknown.
DVD Subtitles: Never render more than one subtitle.