rtsp-stream: Disable multicast loopback for all our sockets
On Windows this is a receiver-side setting, on Linux a sender-side setting. As we provide a socket ourselves to udpsrc, udpsrc is never setting the multicast loopback setting on the socket... while udpsink does which unfortunately has no effect here on Windows but on Linux. https://bugzilla.gnome.org/show_bug.cgi?id=757488