They require no special video decode extension. The push descriptor
extension was a placeholder for when the check was performed inside
the init function.
The issue is that .flush gets called asynchronously, and modifies the
video session state while its being used for decoding. This did not
result in issues since all known vendors do not keep important state
there, but its not compliant with the specs.
Its not necessary to flush the decoder at all when seeking,
so simply don't.
Fixes#20487
