This queries the _ICC_PROFILE property on the root window. It also tries
to reload the ICC when it changes, or if the mpv window changes the
monitor. (If multiple monitors are covered, mpv will randomly select one
of them.)
The official spec is a dead link on freedesktop.org, so don't blame me
for any bugs.
Note that this assumes that Xinerama screen numbers match the way mpv
enumerates the xrandr monitors. Although there is some chance that this
matches, it most likely doesn't, and we actually have to do complicated
things to map the screen numbers. If it turns out that this is required,
I will fix it as soon as someone with a suitable setup for testing the
fix reports it.