It creates a new structure for passing the codec quality structure at _start(),
where it will be filled. The quality level can be set or changed according
encoder limits.
Later the quality level will be set at _update_session_parameters() and at each
frame encoding. That's why it has to be set at _start().
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8007>
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