This currently affects only the xsettings plugin, which needs GDK to use
the x11 backend. It already calls
gdk_set_allowed_backends ("x11") for
this purpose; however, if
GDK_BACKEND=wayland (or any other non-x11
backend possibly) happened to be in the environment, GDK would fail to
initialize at all. This would prevent the xsettings plugin from starting
Big thanks to Olivier Fourdan for figuring this out!