Skip store-overflow for absolute flows during normal traversal.
Put absolute flow display items under their CB.

Skip them when their original parent is being processed.
Figure out that absolute display items can't come before others.

Cos we don't have paint order working yet.
Paint borders before the actual box.

This is a minor fix in lieu of the whole paint order thing (which will
come later).

Before this change, the border was a sibling of the ClipDisplayItem,
which meant that all the children of the block were being painted first,
and then the border was painted on top of them. Bad.