Make sure to create VPx WebCodecs encoders and decoders asynchronously
<rdar://problem/101070923>
Pull request: https://github.com/WebKit/WebKit/pull/5285
Committed 255478@main (c73a0fb7ac16): <https://commits.webkit.org/255478@main> Reviewed commits have been landed. Closing PR #5285 and removing active labels.