Sometimes people make tables with a specific width. In those cases,
we can't just use the auto-sizing algorithm, but instead have to
respect whatever width the content specifies.
This is a bit rickety right now, since we don't implement generation
of anonymous table boxes.
The basic mechanism here is that block layout (which table-cell uses)
now has a virtual way of asking for the width of the logical containing
block. This is necessary as table-row does not produce a block-level
element and so was previously unable to provide a containing block
width for table-cell layout.
If the table has a non-auto specified width, we now interpret that as
a request to use fixed table layout. This will evolve over time. :^)
To make this possible, I also had to give each LayoutNode a Document&
so it can resolve document-specific colors correctly. There's probably
ways to avoid having this extra member by resolving colors later, but
this works for now.
