layout_2020: Paint hoisted positioned fragments in tree order

Instead of painting hoisted position fragments in the order to which
they are hoisted, paint them in tree order and properly incorporate them
into the stacking context.

We do this by creating a placeholder fragment in the original tree position
of hoisted fragments. The ghost fragment contains an atomic id which
links back to the hoisted fragment in the containing block.

While building the stacking context, we keep track of containing blocks
and their children. When encountering a placeholder fragment we look at
the containing block's hoisted children in order to properly paint the
hoisted fragment.

One notable design modification in this change is that hoisted fragments
no longer need an AnonymousFragment as their parent. Instead they are
now direct children of the fragment that establishes their containing block.

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