When layout was split into two phases, floats were laid out as direct
children of the inline formatting context. This meant that they were
positioned properly, but not properly made children of their inline
ancestors' stacking contexts. This change maintains the proper
positioning of floats, but positions them relatively to their inline
ancestors.
The big change here is that `text-align` needs to be taken into account
before actually laying out LineItems. This has the added benefit of
setting inline layout for the implementation of `text-align: full`. Now
all line items are laid out at the real final position and we can adjust
the `start_corner` property of float `BoxFragments` when their ancestors
are laid out.
