When set, river tags and seats’ view titles apply to the output yambar
is on, only.
The default is disabled, which implements the old behavior, where
river tags and seats’ view titles represent the union of all
outputs.
This is mainly to fix a race when river is *not* running; sometimes we
ended up allocating particles for N seats in content(), but then when
iterating the seats, run() had destroyed all, or some of the seats,
causing us to feed NULL pointers to dynlist, which crashed.
