The refactor in bcf7b263 introduced a possible subtraction with overflow
when the statusline is layed out so that the left or right sides are
larger than the padding it would take to align the center area to the
middle.
When the left or right areas are too large, we can evenly space the
elements rather than trying to align the center area to the middle.
This prevents possible underflows and makes sense visually - it's
still easy to tell the areas apart at a glance.