sub: don't read packets if video and audio are disabled

Although disabling both video and audio is surely an obscure corner
case, it's allowed, and we don't want the demuxer to skip arbitrary
packets.

Basically, make the heuristic for checking interleaved files affect
external files only.
sub: fix subtitle timing for TS

The subtitle timing logic always used the demuxer's start time as video
offset. This made external subtitle files "just work" with file formats
like TS, which usually have a non-0 start time. But it was wrong for
subtitles muxed with the TS, so adjust the time offset explicitly with
external files only.