x11: when using --wid, inherit event flags from parent window

When embedding a X window, it's hard to control whether it receives
mouse/keyboard input or not. It seems the X protocol itself makes this
hard (basically due to the outdated design mismatching with modern
toolkits), and we have to take care of these things explicitly.

Simply do this by manually querying and using the parent window event
flags.

This restores some MPlayer behavior (it doesn't add back exactly the
same code, but it's very similar).

This probably has some potential to interfere with libmpv embedding, so
bump the client API minor.

CC: @mpv-player/stable (if applied, client-api-changes.rst has to be
    adjusted to include the 0.5.2 release)
audio/out: prevent burning CPU when seeking while paused

The audio/video sync code in player/audio.c calls ao_reset() each time
audio decoding is entered, but the player is paused, and there would be
more than 1 sample to skip to make audio start match with video start.
This caused a wakeup feedback loop with push.c.

CC: @mpv-player/stable
video: adjust some Matroska 3D formats

There is no proper and exact spec (Matroska tradition), so we probably
have to rely on guessing for this.

Also see issue #1045.
audio: go to draining state instead of EOF if audio starts later

Probably no observable effect, but it's more correct. Setting audio to
EOF could have bad effects otherwise (anywhere the player logic for
example decides whether EOF was reached, and such).