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>