contender is not being shifted enough to hit overlap with master,
probably because of double-spacing and the grid *not* being aligned regularly
despite actually fitting in the gaps - characters move down relative
to the baseline on subsequent lines.

My old code for straghtening didn't just count clear pixels - it used the
sum of black pixels^2 and white pixels^2 - maybe we need that here to detect the
normal-height text lines rather than the abnormally tall spaces between lines.