Playing a disc through the menu means crossing playlist boundaries at
runtime: menu -> title (and back), sub-menu -> sub-menu, title -> title.
Each new playlist can have a different stream count and/or a different
codec mix. The previous slave demuxer was probed once at open and never
refreshed, so after a hop the player either kept decoding stale streams
or saw nothing at all. Tear down and re-open the slave on every nav
discontinuity.