Adaptive sync is a "soft" setting which we degrade of off when not
supported. Some outputs types do not support turning it off (Wayland,
X11), which makes for an awkward three-way test where we first enable,
disable and finally unset the setting.
wlr_output.adaptive_sync_supported tells us whether the output
definitely does not support making changes (backend without support,
connector without the feature), or whether it might work.
Use this to avoid wasting time on adaptive sync test that can never
succeed, and to avoid the Wayland/X11-backend specific unset step.
Kenny Levinsen