player: simplify audio uninit on segment switches

The purpose of temporarily setting stop_play was to make the audio
uninit code to explicitly drain audio if needed. This was the only way
to do it before ao_drain() was made a separate function; now we can just
do it explicitly instead.
manpage: adjust --hwdec docs
vf: minor simplification

Remove the extra vf_chain.output field - there's absolutely no need for
it, because there is always a last filter which will buffer the output.

For some reason, vf_chain.last was never set, which we now need to fix
too.
video: move formatting of image parameters to separate function
mp_image: slightly better image params verbose info
af: check audio params for validity

Normally, these should be valid anyway, so this is just being cautious.
af: remove redundant function
demux: update cache state when paused

This was removed in commit 480f82fa. This caused the cache display not
to update while paused, because the update_cache() function is never
called in the thread (now I remember why the extra call was "needed").

The old implementation intentionally run update_cache() only before
waiting on a mutex, with no further checks for the condition variable.
In theory, this is strictly not sane, but since it was just for the
retrieval of the very fuzzy cache status, it was ok. Now we want to call
update_cache() outside of the mutex though - which means that in order
to avoid missed wakeups, a proper condition has to be used.