Add an experimental feature for letting Xwayland clients scale natively

With the next commits we'll introduce a new mode for scaling of Xwayland apps,
we'll want to put this mode behind an experimental setting though, so add
that setting.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3567>

src/backends/meta-settings.c

@@ -300,6 +300,8 @@ experimental_features_handler (GVariant *features_variant,
         feature = META_EXPERIMENTAL_FEATURE_AUTOCLOSE_XWAYLAND;
       else if (g_str_equal (feature_str, "variable-refresh-rate"))
         feature = META_EXPERIMENTAL_FEATURE_VARIABLE_REFRESH_RATE;
+      else if (g_str_equal (feature_str, "xwayland-native-scaling"))
+        feature = META_EXPERIMENTAL_FEATURE_XWAYLAND_NATIVE_SCALING;
 
       if (feature)
         g_message ("Enabling experimental feature '%s'", feature_str);