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>

data/org.gnome.mutter.gschema.xml.in

@@ -5,6 +5,7 @@
     <value nick="kms-modifiers" value="2"/>
     <value nick="autoclose-xwayland" value="4"/>
     <value nick="variable-refresh-rate" value="8"/>
+    <value nick="xwayland-native-scaling" value="16"/>
   </flags>
 
   <schema id="org.gnome.mutter" path="/org/gnome/mutter/"
@@ -136,6 +137,12 @@
                                         GPU and DRM driver. Configurable in
                                         Settings. Requires a restart.
 
+        • “xwayland-native-scaling”   — lets Xwayland clients use their native
+                                        scaling support. If scaling is not
+                                        supported by client, the client will
+                                        be unscaled. Setting only takes effect
+                                        when “scale-monitor-framebuffer” is
+                                        enabled as well.
       </description>
     </key>
 

src/backends/meta-settings-private.h

@@ -32,6 +32,7 @@ typedef enum _MetaExperimentalFeature
   META_EXPERIMENTAL_FEATURE_KMS_MODIFIERS  = (1 << 1),
   META_EXPERIMENTAL_FEATURE_AUTOCLOSE_XWAYLAND  = (1 << 2),
   META_EXPERIMENTAL_FEATURE_VARIABLE_REFRESH_RATE = (1 << 3),
+  META_EXPERIMENTAL_FEATURE_XWAYLAND_NATIVE_SCALING  = (1 << 4),
 } MetaExperimentalFeature;
 
 typedef enum _MetaXwaylandExtension

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);