layout: Rewrite clipping to be per-display-item instead of having a separate `ClipDisplayItem`.

We push down clipping areas during absolute position calculation. This
makes display items into a flat list, improving cache locality. It
dramatically simplifies the code all around.

Because we need to push down clip rects even for absolutely-positioned
children of non-absolutely-positioned flows, this patch alters the
parallel traversal to compute absolute positions for
absolutely-positioned children at the same time it computes absolute
positions for other children. This doesn't seem to break anything either
in theory (since the overall order remains correct) or in practice. It
simplifies the parallel traversal code quite a bit.

See the relevant Gecko bug:
https://bugzilla.mozilla.org/show_bug.cgi?id=615734

r? @mrobinson