layout: Rewrite display list building to be parallel and to handle
overflow correctly, and opportunistically lay out blocks in parallel
even if floats are present.

This commit fixes the `inline-height-test` in Acid2 by implementing
proper overflow as well as the inline "strut". (See CSS 2.1 § 10.8.1.)
Acid2 was accidentally being rendered properly before because tables'
descendant flows were not being laid out properly.

Display list building is now parallel and is done by bubbling up display
items and layers from parent to child. Speedups of around 60%-70% are
observed on Wikipedia with a 4-core HyperThreaded Core i7. More
optimizations are possible; this is just a start.

To minimize the amount of data that needs to be bubbled up, as well as
to make proper handling of `overflow: hidden` clipping easier, the
`StackingContext` abstraction is now purely internal to the display
list. That is, instead of placing items into a stacking context
directly, display items are placed into display lists alongside a
stacking level. When a stacking context is complete, it is flattened
with the the `flatten` method, which shuffles the display items that
make up the context into their proper order while handling clipping
properly.