Refactor PlatformCALayer to support finishing the delegated layer contents before compositing
<rdar://problem/107605713>
Pull request: https://github.com/WebKit/WebKit/pull/12375
Committed 262613@main (f37c05e1232e): <https://commits.webkit.org/262613@main> Reviewed commits have been landed. Closing PR #12375 and removing active labels.