[WebCodecs] Update WPT validateBlackDots to test 4 pixels instead of just one
<rdar://problem/101540548>
Pull request: https://github.com/WebKit/WebKit/pull/5753
Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/36666
Committed 256017@main (19e96c9ddbe9): <https://commits.webkit.org/256017@main> Reviewed commits have been landed. Closing PR #5753 and removing active labels.