x11: Let X11 connection owners call XSynchronize() themselves

Only make the context carry the boolean state, but move the two libX11
calls into their corresponding connection handler objects.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2444>

src/core/meta-context.c

@@ -245,6 +245,14 @@ meta_context_get_x11_display_policy (MetaContext *context)
   return META_CONTEXT_GET_CLASS (context)->get_x11_display_policy (context);
 }
 
+#ifdef HAVE_X11
+gboolean
+meta_context_is_x11_sync (MetaContext *context)
+{
+  return META_CONTEXT_GET_CLASS (context)->is_x11_sync (context);
+}
+#endif
+
 static gboolean
 meta_context_real_configure (MetaContext   *context,
                              int           *argc,