vf_vapoursynth: add display resolution support

Add two new script environment variables 'video_in_dw' and
'video_in_dh', representing the display resolution of video. Along
with video resolution, sample ratio aspect can be calculated in
scripts.

Currently it's impossible to change sample ratio aspect with single
vapoursynth filter since '_SARNum' and '_SARDen' frame properties
from output clip will be ignored. A following 'dsize' filter is
necessary for this purpose.
vf_vapoursynth: add more VS frame properties

Add the missing frame properties in 48587e88.
manpage: remove duplicated content

Probably be accidentally added in b6b8bffd.

CC: @mpv-player/stable
Signed-off-by: wm4 <wm4@nowhere>
video: minor simplification

This is mostly equivalent, but simpler, and reduces some duplication.
video: move some code around

No functional changes. init_vo() is now needed a bit further down, and
moving it keeps definition and use close. adjust_sync() will be used by
a function further up in one of the following commits.
video: get rid of video_next_pts field

Not really needed anymore. Code should be mostly equivalent.

Also get rid of some other now-unused or outdated things.
video: refactor queue handling

This simplifies the code, and fixes an odd bug: the second-last frame
was displayed for a very short duration if framedrop was enabled. The
reason was that basically the time difference between second-last and
last frame were skipped, because at this point EOF was already
signaled. Also see commit b0959488 for a similar issue in the
same code.

This removes the messiness of the next_frame 2-frame queue, and
strictly runs the "new frame" code when a frame is moved to the first
position of the queue, instead of somehow messing with return codes.

This also merges update_video() into video_output_image().
player: don't clobber playback position on video end

If video reaches EOF, and audio is also EOF (or is otherwise not
meaningful, like audio disabled), then the playback position was briefly
set to 0. Fix this by not trying to use a bogus audio PTS.

CC: @mpv-player/stable (maybe)