Needs a CountedElement in order to distinguish the case of the element
not being present and the element being present with a value of zero.

(It has been argued by Ridley Combs that one should only ever use the
AV_DISPOSITION_DUB field for audio tracks. Yet given that there is no
definition for the disposition flags, one can also interpret it to mean
that e.g. a subtitle track is meant to be used with the dubbed audio
track or the original audio track. This commit interprets this flag in
this sense, which also allows to maintain it on remuxing.)
Reviewed-by: Anton Khirnov <anton@khirnov.net>
Reviewed-by: Ridley Combs <rcombs@rcombs.me>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>