lavr: fix mixing matrix reduction when normalization is disabled

In some cases when an input contributes fully to the corresponding output, other inputs may also contribute to the same output. This is the case, for example, for the default 5.1 to stereo downmix matrix without normalization.

This commit fixes 148d6384a30a2ff50742f3774d7dd64e53faa103.
Fix: Play > Subtitles menu sometimes showed the wrong track as selected.

If the subtitles track was changed at the splitter level the Play >
Subtitles menu wasn't reflecting the change correctly.

The handling of the subtitles tracks have been rewritten to avoid storing the track number.