xvimagesink: don't recreate xvcontext

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