tagdemux: Use upstream GST_EVENT_STREAM_START if present

If we have an upstream GST_EVENT_STREAM_START, use that one instead
of creating a new one which could be completely different from the
upstream one and drop information (like the stream flags and stream
object).

Only create a new event if we don't already have one from upstream

https://bugzilla.gnome.org/show_bug.cgi?id=797215