sub: load .mks files as external subtitles
audio/out/push: when using audio wait fallback, recheck condition

If calling ao->driver->wait() fails, we need to fallback to timeout-
based waiting. But it could be that at this point, the mutex was already
released (and then re-acquired). So we need to recheck the condition in
order to avoid missed wakeups.

This probably wasn't an actually occurring problem, but still could
cause a small race-condition window if the dynamic fallback is actually
used.
ao_oss: wait for events with poll()

The intention is to avoid using the timeout-based fallback.

There's some minor hope that this will help with OpenBSD (see #1239),
although it probably won't.

Some chance that this will cause trouble with obscure OSS
implementations or emulations.
command: add display-names property

Add VOCTRL_GET_DISPLAY_NAMES and call it when the property is
requested. The vo should return the names of the displays that the mpv
window is covering.  For example, with x11 vos, xrandr names LVDS1,
HDMI1, etc.
vo/x11: implement VOCTRL_GET_DISPLAY_NAMES with xrandr names (e.g., "LVDS1")

XRRGetOutputInfo contains a "name" element which corresponds to to the
display names given to the user by the "xrandr" command line
utility. Copy it into the xrandr_display struct for each display.
On VOCTRL_GET_DISPLAY_NAMES, send a copy of the names
of the displays spanned by the mpv window on.