audio: minor improvements to timeline switching

In theory, timestamps can be negative, so we shouldn't just return -1
as special value.

Remove the separate code for clearing decode buffers; use the same code
that is used for normal seek reset.
player: restore silent seeking

Commit 846257da introduced an accidental feature: if you kept seeking
(so playback never really resumes), the audio would never be played.
This was nice, but commit 4c25b000 accidentally removed it again (due
to the video_next_pts being earlier available than it used to be, so
audio could be played before the player executed the next queued seek).

Implicitly reintroduce the old behavior again by not decoding a second
video frame immediately. Usually, the second frame is used to compute
the frame duration needed to for accurate framedropping, but since the
first frame after a seek is never dropped, we don't need this.

Now the video code will queue the new frame to the VO immediately, and
since fill_audio_out_buffers() is called in the playloop before
write_video() and execute_queued_seek(), it never gets the chance to
enter STATUS_READY, and seeks will be silent.

This also has a nice side-effect: since the second frame is not decoded
and filtered, seeking becomes slightly faster (back to the same level
as with framedrop disabled).

It seems this still sometimes plays a period of audio when keeping a
seek key down. In my tests, this appeared to happen because the seek
finished before the next key repeat was sent.
video: avoid unnecessary frame dropping

If duration<0, it means the duration is unknown. Disable framedropping,
because end_time makes no sense in this case.

Also, strictly never drop the first frame.

This fixes weird behavior with the cover-art case (for the 100th time).