Add a (broken) Acid2 reftest. Fix #1993

**Do not merge yet.** This new test is very much intermittent, at least on my machine. Just as frequently as it passes, it test sends Servo in an infinite loop or just gives an empty (white) output (and obviously fails.)

@pcwalton, any idea what we could do about this?

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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 }

This works around the lack of layer pixel snapping. (See #2035)

The reference is modified to match the current rendering of the test:

* Rows 1, 2 (forehead), 4, and 5 (eyes) are missing.
* The yellow background in row 3 overflows by 1px at the bottom.
* The top-right and bottom-left borders of the nose are 1px to high.
* Row 12’s height is 3px more than it should be.
* Rows 13 and 14’s positions are 4px lower than they 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.

Adding `.chin div { font-size: 3px }` works around the chin issue (rows 12 to 14) for reasons that escape me yet.