plugins: Unset GDK_BACKEND envvar if a specific backend is needed
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
up.
Big thanks to Olivier Fourdan for figuring this out!
Bug: https://bugzilla.redhat.com/show_bug.cgi?id=2022283 Bug: https://bugs.debian.org/1008992