command: force video refresh when changing aspect

Pausing the player and changing the aspect would leave the VO without a
frame to display.
demux_mkv: don't add too many subtitle packets during seeking

In insane files with a very huge number of subtitle events, and if the
--demuxer-mkv-subtitle-preroll option is given, seeking can still
overflow the packet queue. Normally, the subtitle_preroll variable
specifies the maximum number of packets that can be added. But once this
number is reached, the normal seeking behavior is enabled, which will
add all subtitle packets with the right timestamps to the packet queue.
At this point the next video keyframe can still be quite far away, with
enough subtitle packets on the way to overflow the packet queue.

Fix this by always setting an upper limit of subtitle packets read
during seeking. This should provide additional robustness even if the
preroll option is not used.

This means that even with normal seeking, at most 500 subtitle packets
are demuxed. Packets after that are discarded.

One slightly questionable aspect of this commit is that subtitle_preroll
is never reset in audio-only mode, but that is probably ok.
mplayer: read subtitle packets as soon as possible

Call update_subtitles() on every iteration of the playloop, so that
subtitle packets are read as soon as possible, instead of every time a
video frame is displayed. This helps in case the packet queue is swamped
with subtitle packets, which can happen with certain insane mkv files.
The change will simply cause the subtitle queue to be emptied on each
playloop iteration.

The timestamps update_subtitles() uses for display are the same before
and after this commit. (Important for files which have subtitle packets
with timestamps or duration not set.)