mplayer: fix --step EOF handling
mplayer: fix framestepping on ordered chapter segment boundaries

Trying to step over a segment boundary didn't work, and the video was
stuck at the end of the current chapter. At this point, both video and
audio of the segment has ended, and the segment switching code is going
to call seek() to go to the next segment (the part of the code in
run_playloop that uses end_is_chapter). However, this seek() is not
called if playback is paused, and the framestepping code always paused
before this code is run.

Move the framestepping code below the chapter switching code. The added
restart_playback condition makes sure the code is called only after at
least one video frame has been shown. Also don't reset the framestep
counter after seek. It's not needed, and removing it prevents full
unpausing when stepping over a segment boundary.

This also terminates playback when frame stepping at the end of the
file. The --keep-open option can be used to get the old behavior.
mplayer: switch back to video PTS for reporting playback time

The main problem with video PTS was that it wasn't very useful when
playing audio files with cover art. Using the audio time instead was an
obvious solution. Unfortunately, this leads to "inexact" reporting of
the playback time in paused mode, and audio is always ahead by small,
essentially random amounts of time ahead. This is possibly because the
times reported by AOs are not entirely accurate when paused (see commit
9b3bf76).

Switch back to video PTS, and use a simpler way to deal with the cover
art case: if the video has ended, use the audio PTS.

Also see commit f9a259e (and the commits referenced from there).
command: silence "Audio: no audio" line for playback speed

If no audio stream is selected, this line will be printed by
reinit_audio_chain() when changing playback speed.
demux_mkv: fix handling of 0 DisplayWidth/Height

Commit 546ae23 fixed aspect ratio if the DisplayWidth or DisplayHeight
elements were missing. However, some bogus files [1] can have these
elements present in the file, but set to 0. Use 1:1 pixel aspect for
such files.

[1] https://ffmpeg.org/trac/ffmpeg/ticket/2424