To recap: the original XC-SECURITY extension disallowed background "None" if
the window was untrusted. XACE 1.0 preserved this check as a hook function.
XACE pre-2.0 removed the hook and first abolished background "None entirely,
then restored it as a global on/off switch in response to Bug #13683.
Now it's back to being per-window, via a flag instead of a hook function.