When the drm video output is used under VT with a terminal multiplexer
like screen, the VT_SETMODE ioctl call will fail since the controlling
terminal is a pseudoterminal instead of a real one, thus the VT switcher
will be unavailable.
The problem is, the wait_events function inside vo_drm.c will test for
this, and will do nothing if the switcher is not active. Normally, this
would not be noticed, however, when you pause a video, or if you're playing
multiple image files, mpv will suddenly start busy waiting on a single
core.
I confirmed this by building mpv with gprof support, in a few seconds,
wait_events got called about 90 million times.
So I added a fallback, when the VT switcher is not availble, just use the
vo_wait_default function. I tested it and it's working well so far.
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