This fixes another problem similar to the one fixed in be1854c617 (Avoid
UI functions in non-main thread in wxGStreamerMediaBackend, 2020-09-02)
and does it in a similar way: SetupXOverlay(), which calls gdk_flush(),
was called from gst_bus_sync_callback() which is executed in the worker
thread, so use CallAfter() to call it from the main thread later.