core/debug-control: Introspect it to export the dbus service with lg

by executing `global.context.get_debug_control().exported = true`.

This makes it possible to get access to the debug service without having
to start with `--enable-debug`.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3902>

src/core/meta-context.c

@@ -884,6 +884,12 @@ meta_context_init (MetaContext *context)
     }
 }
 
+/**
+ * meta_context_get_debug_control:
+ * @context: The #MetaContext
+ *
+ * Returns: (transfer none): the #MetaDebugControl
+ */
 MetaDebugControl *
 meta_context_get_debug_control (MetaContext *context)
 {