This is a feature that was never properly implemented in the previous
layout system. We still need to preserve their in-tree order in the
display list though.
This adds very rudimentary support for paint order in stacking context.
In particular z-index is now handled properly, apart from issues with
hoisted fragments.
This adds an intermediary data structure that allows the display list
builder to move through the fragment tree in stacking context painting
order. Spatial nodes are built during this phase and all display list
items are added to the end of the display list.
This makes it so that position:fixed elements do not scroll with the
rest of the contents, but does not tackle the rest of the issues with
their positioning.
