[css-transforms-2] add a more formal definition of flattening

[dbaron]

I noticed that css-transforms-2 doesn't have a formal definition of flattening. While writing one (with the intent of linking to it), I also noticed that nothing actually says that 3-D scenes are eventually flattened at the root of the 3D Rendering Context.

I have edits to add this, but since they're somewhat substantive I'm filing this issue for review of those edits.

cc @smfr @mattwoodrow

[dbaron]

Actually, on second thought, I've reverted the edit that I made. I still think it would probably be useful to say what is meant by flattening, but I think what I wrote contradicted § 4.2. Processing of Perspective-Transformed Boxes, so I thought it was better to remove it.