Table row layout is now split into two phases:
1. Compute all the column widths (even taking colspan into account!)
2. Place all cells at the correct x,y offsets based on column widths.
Both phases visit all rows and all cells.
The box tree and line boxes now all store a relative offset from their
containing block, instead of an absolute (document-relative) position.
This removes a huge pain point from the layout system which was having
to adjust offsets recursively when something moved. It also makes some
layout logic significantly simpler.
Every box can still find its absolute position by walking its chain
of containing blocks and accumulating the translation from the root.
This is currently what we do both for rendering and hit testing.
