layout: Implement `opacity` per CSS-COLOR § 3.2.

This adds the infrastructure necessary to support stacking contexts that
are not containing blocks for absolutely-positioned elements. Our
infrastructure did not support that before. This minor revamp actually
ended up simplifying the logic around display list building and
stacking-relative position computation for absolutely-positioned flows,
which was nice.

This will need this PR: https://github.com/servo/rust-azure/pull/112 I have not updated the Cargo.lock file yet because I want the merge commit.

r? @glennw
f? @SimonSapin