The picture parameter picture->top_field_first is reused in this mode
to signal the TOP fields. As a side effect, it will change every frame
and current code assumed that if this changes then a renegotiation is
needed. Fixed this by ignoring that change whenever we are decoding one field
only.
Fixes#1523
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3276>