settings: Allow overriding experimental settings with an env var

This makes it simpler to experiment with experimental settings without
having to make changes to any gsettings fields one might not want to
change.

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

src/backends/meta-settings.c

@@ -43,6 +43,14 @@ enum
   N_SIGNALS
 };
 
+static GDebugKey experimental_feature_keys[] = {
+  { "scale-monitor-framebuffer", META_EXPERIMENTAL_FEATURE_SCALE_MONITOR_FRAMEBUFFER },
+  { "kms-modifiers", META_EXPERIMENTAL_FEATURE_KMS_MODIFIERS },
+  { "autoclose-xwayland", META_EXPERIMENTAL_FEATURE_AUTOCLOSE_XWAYLAND },
+  { "variable-refresh-rate", META_EXPERIMENTAL_FEATURE_VARIABLE_REFRESH_RATE },
+  { "xwayland-native-scaling", META_EXPERIMENTAL_FEATURE_XWAYLAND_NATIVE_SCALING },
+};
+
 static guint signals[N_SIGNALS];
 
 struct _MetaSettings
@@ -544,6 +552,8 @@ meta_settings_dispose (GObject *object)
 static void
 meta_settings_init (MetaSettings *settings)
 {
+  const char *experimental_features_env;
+
   settings->interface_settings = g_settings_new ("org.gnome.desktop.interface");
   g_signal_connect (settings->interface_settings, "changed",
                     G_CALLBACK (interface_settings_changed),
@@ -567,6 +577,21 @@ meta_settings_init (MetaSettings *settings)
   g_signal_connect (settings, "ui-scaling-factor-changed",
                     G_CALLBACK (meta_settings_update_font_dpi), NULL);
 
+  experimental_features_env = getenv ("MUTTER_DEBUG_EXPERIMENTAL_FEATURES");
+  if (experimental_features_env)
+    {
+      MetaExperimentalFeature experimental_features;
+
+      experimental_features =
+        g_parse_debug_string (experimental_features_env,
+                              experimental_feature_keys,
+                              G_N_ELEMENTS (experimental_feature_keys));
+
+      meta_settings_override_experimental_features (settings);
+      meta_settings_enable_experimental_feature (settings,
+                                                 experimental_features);
+    }
+
   update_global_scaling_factor (settings);
   update_experimental_features (settings);
   update_xwayland_grab_access_rules (settings);