[css-transforms] Unify the definition of transform-style & 3D context

Oops, forgot subject in the last one.

<style> div { transform-style: preserve-3d; } </style>
> <div id="A" style="background:some_image; translateZ(0);">
>     Foreground
>     <div id="B" style="transform:translateZ(0); position:relative;
> z-index:-1;">Stuck in middle?</div>
> </div>


> Should B to stack in between the background/foreground of A because it
> has negative z-index? I think the sensible thing to do is to pop it to
> its own plane.


This all seems easier to reason about if we just consider anything with 3D
transforms (even if identity transforms) a separate plane. Thinking about
the B plane being sandwiched between content of the AC plane's texture is
more cognitively complex (as well as more complex in implementation).
Popping it out, and leaving it out sounds good.

*/#!/*JoePea