codecs: Signal required DPB size for AV1,MPEG2,VP8, and VP9 via new_sequence()

codecs: Signal required DPB size for AV1,MPEG2,VP8, and VP9 via new_sequence()

Make all codecs consistent so that subclass can know additional DPB
size requirement depending on render-delay configuration regardless
of codec. Note that render-delay feature is not implemented for AV1
yet but it's planned.

Also, consider new_sequence() is mandatory requirement, not optional

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2343>