Filter and sort DVD subtitle tracks so that only the tracks mapped into the current program chain are shown.

This patch also adds the subtitle streams that are mapped but have not yet been
physically read.  This allows the user to select the language without having to
wait for the first subtitle packet (and therefore miss it).

Fixes #9429

Signed-off-by: Jonatan Lindblad <jlindblad@mythtv.org>
Merge branch 'master' of code.mythtv.org:mythtv