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-private.h

@@ -49,6 +49,10 @@ struct _MetaContextClass
                                     GError      **error);
 
   void (* notify_ready) (MetaContext *context);
+
+#ifdef HAVE_X11
+  gboolean (* is_x11_sync) (MetaContext *context);
+#endif
 };
 
 const char * meta_context_get_name (MetaContext *context);
@@ -64,4 +68,9 @@ MetaWaylandCompositor * meta_context_get_wayland_compositor (MetaContext *contex
 
 MetaX11DisplayPolicy meta_context_get_x11_display_policy (MetaContext *context);
 
+#ifdef HAVE_X11
+META_EXPORT_TEST
+gboolean meta_context_is_x11_sync (MetaContext *context);
+#endif
+
 #endif /* META_CONTEXT_PRIVATE_H */