Add a (broken) Acid2 reftest. Fix #1993

The rendering is incorrect, but this should help catch further regressions.

The test is modified to add the following CSS:

    .intro { display: none }
    html #top { margin-top: 0 }

To disable the "scrolling" part of the test.

The reference is modified to match the current rendering of the test:

* Rows 4 and 5 (eyes) have a red background.
* The nose is not quite where it should be.
* Row 12’s height is 3px more than it should be.

(See http://www.webstandards.org/action/acid2/guide/ for row numbers.)

The nose issue seems be related to rounding vs truncating
when pixel-snapping borders, but this is only a guess.

The test is marked `flaky_gpu` since the nose rendering
is slightly different on CPU and GPU.