c784820454 introduced a bool option type
as a replacement for the flag type, but didn't actually transition and
remove the flag type because it would have been too much mundane work.
See https://sw.kovidgoyal.net/kitty/graphics-protocol/
This makes no attempt at querying terminal features or handling
terminal errors, as it would require mpv to pass the response codes
from the terminal to the vo instead of interpreting them as
keystrokes made by the user and acting very unpredictably.
Tested with kitty and konsole.
Fixes#9605
