xvimagesink: don't recreate xvcontext
A xvcontext can be created early in gst_xvimagesink_set_window_handle(). In this case don't recreate, i.e. overwrite it in gst_xvimagesink_open(). Otherwise XEvents won't be handled in the xevent listener thread. Fixes a regression when setting the window handle on the sink in the very beginning before changing its state. https://bugzilla.gnome.org/show_bug.cgi?id=715138