The name "initial containing block" was wrong for this, as it doesn't
correspond to the HTML element, and that's specifically what it's
supposed to do! :^)
If a box has clearance and margin bottom of preceding box is greater
than static y of the box then it should also affect y offset in current
block container so subsequent boxes will get correct y position too.
Though table wrappers are anonymous block containers (because
TableWrapper is inherited from BlockContainer) with no lines they
should not be skipped in block auto height calculation.
