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.
