Ensure gtk apps only run on X11

dialog-gtk3/src/main.rs

@@ -77,6 +77,14 @@ fn main() -> anyhow::Result<()> {
     init_logging("BSCREENSAVER_DIALOG_GTK3_LOG");
     glib::log_set_default_handler(glib::rust_log_handler);
 
+    // Can't use the rust version as it requires gtk_init() to be
+    // called first, but the underlying C function requires that
+    // it hasn't.
+    let backends = CString::new("x11").unwrap();
+    unsafe {
+        gtk::gdk::ffi::gdk_set_allowed_backends(backends.as_ptr());
+    };
+
     let config = Configuration::load()?;
     let new_login_command =
         if config.new_login_command == NewLoginCommand::Disabled {