Only the last frame of a super frame should be displayed by default. When
converting from super frame to frame, mark all frames as decode only except the
last one. This fixes vp90-2-22-svc_1280x720_3.ivf conformance test with
stateless decoders such as VA.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8944>