multiqueue: use new stream-start event sparse flag to avoid overreading subtitles

multiqueue: use new stream-start event sparse flag to avoid overreading subtitles

This will mean a much lower delay before a subtitles track changes take
effect. Also avoids excessive memory usage in many cases.

This will also consider sparse streams as (individually) never full, so
as to avoid blocking all playback due to one sparse stream.

https://bugzilla.gnome.org/show_bug.cgi?id=600648