recorder: always mux all packets on discont/close

This is the muxer used by all 3 stream recording features (why are there
so many?). It tried hard to avoid writing broken files. In particular,
it buffered packets until it new there was a keyframe packet (which, in
mpv's/FFmpeg's definition, mean seek points from which decoding can
resume), or final EOF. The danger that was probably considered here was
that due to video frame reordering, not muxing some trailing, missing
packets of a keyframe range could lead to broken decoding or skipped
frames, so better discard packets belonging to an incomplete range.
Sounds like a good idea so far.

Unfortunately, this will drop an entire keyframe range even if the
current packet run is complete and mp_recorder_mark_discontinuity() is
called, simply because recorder.c can not know that the next packet
would have been a keyframe.

It seems better to mux all packets to avoid losing valid data, even if
it means that sometimes packets/frames will be missing from the file. It
benefits especially the dump-cache command, which will call the function
to signal a discontinuity after every range. Before this commit, it
discarded the last packets, even if they were perfectly fine.

(An alternative solution for dump-cache would have been a second
discontinuity marker function, that communicates that the current packet
range is complete. But this commit's solution is simpler and overall
more robust, at the danger of producing more semi-broken files.)

This may make some of the complex buffering/waiting logic in recorder.c
pointless.

Untested (in this final form).

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