We now produce a `matrix3d()` value when appropriate.
Some sites (such as gsap.com) request the resolved style for `transform`
when there's no viewport paintable, but the element itself does already
have a stacking context. This fixes crashes in that case, because we now
do not access the stacking context at all.
We also do not wrap the result as a StyleValueList any more. The
returned StyleValue is only serialized and exposed to JS, so making it a
StyleValueList has no effect.