Notice that the expansion of the line width depends on the order of the
points:
* If the bigger axis value is provided as the *second* point the line
expand first to the *positive* side of the axis.
* If the bigger axis value is provided as the *first* point the line
expand first to the *negative* side of the axis.
* If the line's width is odd this doesn't matter, as the line will
expand the same amount to both sides.
This behavior should be consistent in both horizontal and vertical lines.
