Add debug controller

The debug controller can optionally, when passing --debug-control,
enable manipulating debug state, so far enabling/disabling HDR, via
D-Bus.

It's always created, in order to have a place to store debug state and
emit signals etc when it changes, but so far, it doesn't have its own
state it tracks, it just mirrors that of the monitor manager.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3432>

src/core/meta-context.c

@@ -103,6 +103,8 @@ typedef struct _MetaContextPrivate
 #ifdef HAVE_WAYLAND
   MetaServiceChannel *service_channel;
 #endif
+
+  MetaDebugControl *debug_control;
 } MetaContextPrivate;
 
 G_DEFINE_TYPE_WITH_PRIVATE (MetaContext, meta_context, G_TYPE_OBJECT)
@@ -329,7 +331,13 @@ meta_context_real_configure (MetaContext   *context,
     }
 
   option_context = g_steal_pointer (&priv->option_context);
-  return g_option_context_parse (option_context, argc, argv, error);
+  if (!g_option_context_parse (option_context, argc, argv, error))
+    return FALSE;
+
+  priv->debug_control = g_object_new (META_TYPE_DEBUG_CONTROL,
+                                      "context", context,
+                                      NULL);
+  return TRUE;
 }
 
 /**
@@ -747,6 +755,8 @@ meta_context_dispose (GObject *object)
 
   g_clear_pointer (&priv->backend, meta_backend_destroy);
 
+  g_clear_object (&priv->debug_control);
+
   g_clear_pointer (&priv->option_context, g_option_context_free);
   g_clear_pointer (&priv->main_loop, g_main_loop_unref);
 
@@ -839,3 +849,11 @@ meta_context_init (MetaContext *context)
         g_warning ("Failed to save the nofile limit: %s", error->message);
     }
 }
+
+MetaDebugControl *
+meta_context_get_debug_control (MetaContext *context)
+{
+  MetaContextPrivate *priv = meta_context_get_instance_private (context);
+
+  return priv->debug_control;
+}