This patch just adds the new root paintable and updates the tests
expectations. The next patch will move painting logic from the layout
viewport to the paint viewport.
Allow the left margin of a box which creates a block formatting context
to overlap with left floating boxes which are siblings in the document
tree.
Fixes#20233 and the comment layout on https://lobste.rs.
The margin from the containing blocks shouldn't be included in the
amount by which we increment x after a float was places. That coordinate
should be relative to the containing block.
Fixes the comments layout on https://lobste.rs.
