x11: don't wait for asynchronous X resizing with window-scale

Instead of letting the window-scale property return the old value until
X11 actually executed the resize, just set the new assumed internal
window size immediately. This avoids a "lag" between setting and reading
the window-scale property, like OSD controls typically do.

Remove the additional calls from vo_x11_highlevel_resize() - they're
pointless and slightly wrong, and resize events will take care of
updating these things correctly anyway.

Fixes #1176.

("window-scale" works via VOCTRL_[S|G]ET_UNFS_WINDOW_SIZE.)
vf_vapoursynth: add debug message when returning error from GetFrame

Some filters still (or will) behave badly on these errors, so explicitly
log when it happens.
demux_mkv: fix undefined behavior

With some files, the extradata variable can remain uninitialized, but
will be used for memory access.

CC: @mpv-player/stable (with high priority)
audio/out: add "auto" pseudo-device

Also, don't set an empty string for the fallback device if an AO doesn't
list any devices.
ao_openal: implement device listing
ao_portaudio: implement device listing
ao_dsound: implement device listing
ao_wasapi: implement device listing