[css-transforms-2] Clarify interaction with mix-blend-mode

[mattwoodrow]

This testcase uses preserve-3d and mix-blend-mode, and doesn't contain any grouping property values on the element with preserve-3d.

It does however render with a transform-style used value of flat, in both blink and gecko.

In gecko the effective order of operations (which I don't think are specified) are as follows:

Transform(parent)
  Isolate(parent)
    BlendMode(child)
      Transform(child)
        BackgroundColor(child)

The presence of both the isolation, and the blend-mode interrupt the transform chain and disable preserve-3d, but neither of those are mentioned as grouping property values (mix-blend-mode is a grouping property on the element with preserve-3d, but that's not the problem here).

Assuming this order of operations is correct, then I think we want to remove mix-blend-mode as a grouping property (since it's logically outside the transform), and instead make the case where we've isolated for a descendant mix-blend-mode a grouping 'property'.

I'm not really sure how to specify that (since it's not a property as such, but a pretty complex set of states).

It's also interesting that compositing-1 says that all stacking contexts are isolated, which seems to be a conflict with the definition of grouping properties (which I understand as the subset of stacking contexts that really require isolation).

@dbaron Does this match your understanding of how blink handles this?