The start and end points of the vertical and adjacent horizontal borders
were apart by 1.5 to 3 twips, leading to small visible gaps in the corners.
This is fixed by using lcl_AlignWidth/lcl_AlignHeight on the border widths,
and by always computing the start/end positions from the outer edge,
which makes the horizontal and vertical start/end points match.
(regression from 0f0896c2)