Optimize CSS images by caching StyleImages when possible
<rdar://problem/104654181>
Pull request: https://github.com/WebKit/WebKit-security/pull/17
Pull request: https://github.com/WebKit/WebKit/pull/9144
Committed 259538@main (766d0eca343b): <https://commits.webkit.org/259538@main> Reviewed commits have been landed. Closing PR #9144 and removing active labels.