layout: Implement `background-position` for fixed lengths.

Needed for Acid2 lines 4 and 5 (eyes).
layout: Implement `background-repeat`.

Needed for Acid2 lines 4, 5, and 12.
layout: Parse `background-attachment: fixed`.

Needed for Acid2 line 12.
layout: Propagate floats around properly inside absolutely positioned
elements.

Needed for Acid2, and makes Wikipedia look a lot better!