Gstreamer videos always play on top, popups are hidden from the video

I use QtWidget in my program and it plays video launching gstreamer on top of the program. The device is an arm imx53 that uses the pluging mfw_v4lsink to display video (i.e. use Video4Linux of gst-fsl-plugin). The problem is that the video is always visible on display and no widget-application can hide it. There is a way, modifying the plugin sources, to change this behaviour (Video visible only if the widget is transparent or absent)?
Thanks to everybody can give me a solution.