Remove lower alignment zone at +1 pixel

Remove lower alignment zone at +1 pixel

I think I added it because a lot of mathematical operators (like '+')
have their bottom edge there.  But such operators don't really need
their bottom edges aligned: it's much more important that they should
have their proper shapes and should have their centre lines aligned.

This only slightly improves the rendering of '+' in FreeType 2.12.1:
it's still asymmetric but the asymmetry is in a better direction.
Moreover, some glyphs with descenders like "section" are substantially
improved.