Proxy the force-live and min-upstream-latency propertyies to the internal glvideomixerelement at construction time. force-live has to be set during construction of the glvideomixerelement, so that has to be deferred until the _constructed() call. Make sure that all other existing proxied properties will still get set once the element is created. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4494>