[csswg-drafts] Pull Request: [css-transforms-2] Revert to the TR rendering model, where transform-style:preserve-3d establishes the 3d rendering context. #1950

mattwoodrow has just labeled a pull request from mattwoodrow for https://github.com/w3c/csswg-drafts as "css-transforms-2":

== [css-transforms-2] Revert to the TR rendering model, where transform-style:preserve-3d establishes the 3d rendering context. #1950 ==
This changes the 3d rendering model to be closer to the old TR (and blink/gecko's implementation), where transform-style:preserve-3d is what establishes/extends a rendering context, rather than flat.

It also makes transform-style:preserve use parent elements instead of containing blocks (or stacking contexts) to resolve handling of overflow:hidden (which is neither).

See https://github.com/w3c/csswg-drafts/pull/3750