[gnome-settings-daemon] plugins: Unset GDK_BACKEND envvar if a specific backend is needed

[Bastien Nocera <hadess src gnome org>]

commit c960ce46125f819379fe659883dafa0087d24fe3
Author: Michel Dänzer <mdaenzer redhat com>
Date:   Wed Jul 6 18:31:37 2022 +0200

    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

 plugins/common/daemon-skeleton-gtk.h | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/plugins/common/daemon-skeleton-gtk.h b/plugins/common/daemon-skeleton-gtk.h
index 596281df..4e67706e 100644
--- a/plugins/common/daemon-skeleton-gtk.h
+++ b/plugins/common/daemon-skeleton-gtk.h
@@ -229,6 +229,9 @@ main (int argc, char **argv)
        }
 
         gdk_set_allowed_backends (GDK_BACKEND);
+
+        /* GDK would fail to initialize with e.g. GDK_BACKEND=wayland */
+        g_unsetenv ("GDK_BACKEND");
 #endif
 
         error = NULL;