Make collectDescendantLayersAtPoint() something we can re-use for UI-side compositing
<rdar://problem/102741971>
Pull request: https://github.com/WebKit/WebKit/pull/6901
Committed 257135@main (2d88fb593587): <https://commits.webkit.org/257135@main> Reviewed commits have been landed. Closing PR #6901 and removing active labels.