Gst.init() has to be called before GstPbutils is imported
This makes sure that we do not try to use GstPbutils before Gst is init
and in case GstPbutils is imported while Gst is not imported, use the
GstPbutils.pb_utils_init()
function to have the oportunity to
initialize the overrides.
Not that we also introduce a GstPbutils.init()
variant because
GstPbutils.pb_utils_init()
is an ugly name.