player: don't reset buffering pausing state on seeks

This was added with commit 3cbd79b3, but it turns out this
unintentionally enables "real" pausing when seeking while buffering. It
was done for ensuring correct state of the "cache-buffering-state"
property, but it also turns out that this was unneeded (another variable
that is reset when seeking happens to take care of this).
video: try harder to decode cover art picture only once

For cover art, we pretend that the video stream is infinite, but also
stop decoding once we have an image on the VO (this seems advantageous
for the case when strange filters are inserted or the VO image gets
lost). Since a while ago, the video chain started decoding 2 images
though ("Non-monotonic video pts: 0.000000 <= 0.000000"), which is
annoying and wasteful.

Improve this by handling a certain corner case at initialization, which
will decode a second image while the first one is still stuck in the
filter chain. Also, just in case there are filters which buffer a lot,
also force EOF filtering (which means we tell the filters to flush
buffered frames).

CC: @mpv-player/stable
vf_lavfi: proper rounding for lavfi->mpv aspect ratio

Or so I think. Not like it matters anyway.
client API: rename --input-x11-keyboard to --input-vo-keyboard

Apparently we need this for Cocoa too. (The option was X11 specific in
the hope that only X11 would need this hack.)
manpage: improve --wid description

In particular, add a basic description of how Cocoa embedding works.