ShadowBlur::blurLayerImage: fix out-of-bounds subspan creation and wrong comparison
rdar://130755880
Committed 281274@main (7bc1ae1abbb9): <https://commits.webkit.org/281274@main> Reviewed commits have been landed. Closing PR #31134 and removing active labels.
Committed 280938.116@safari-7619-branch (cc352ba68a0b): <https://commits.webkit.org/280938.116@safari-7619-branch> Reviewed commits have been landed. Closing PR #1482 and removing active labels.