screenshot: change %w format to be more like %t

Instead of containing a format string within %w{...}, simply allow %w
to specify one item of a time format string. This is simpler, more like
other format specifiers (%t),  and probably easier to use too.
manpage: remove version and date fields

These would have to be updated manually all the time. Replacing them
automatically would be possible, but additional work, and would force
regeneration of the manpage way too often.

We decided that we don't need these fields.
Add mpv.desktop, let Makefile install the desktop file and the icons

The mpv.desktop file is taken from the Arch package [1]. It appears to
be based on the mplayer2 git mplayer.desktop file (e.g. very similar
MimeType field), with minor modifications applied by Arch package
maintainers.

Note that for now, this doesn't show a terminal (Terminal=false), which
might not always be ideal. For example, if the file is audio only, or
if VO initialization fails for some reason, mpv will run in the
background and play audio without showing a window. But users prefer
running it without terminal, and don't want to play audio files with
it.

Maybe a --force-window option will be added in the future, which would
always create a VO window, and compensate for these issues.

[1] https://aur.archlinux.org/packages/mpv-git/
manpage: remove bogus space in example usage
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.)
gl_common: signal to GL backend whether we are probing

This is supposed to reduce the amount of useless error messages shown
during initialization of vo_opengl. If multiple backends are compiled,
usually only one of them will work. For example, on Linux both X and
Wayland backends can be compiled, but usually either Wayland or X is
running. Then, if Wayland is not running, but X is, trying to initialize
the Wayland backend should not spam the terminal with error messages.

Code using this should probably raise the log level for error messages
from MSGL_ERR to MSGL_V. Only error messages that are typically printed
in these situations should be changed. For example, if Wayland is
available, but some other part of initialization fails, the error
messages should probably be visible.
x11_common: probing

Not too sure about this. It sucks a bit that I had to add vo.probing.
wayland/common: probing
gl_common: add wayland backend before x11 backend
x11/common: print message according to probing status
wayland/common: print message according to probing status
Removed duplicating message-call in x11/common and wayland/common
wayland/common: use MP_MSG rather than mp_msg
gl/common: removed unused field
Remove unnecessary check if dispName is empty