Adjust bounds fiddling to handle text with leading whitespace. Fixes #19.

We previously eliminated leading whitespace because some fonts stick it in
there just for kicks. But that also eliminated leading whitespace in text like
" foozle". So we just let the leading whitespace lie whether it be intentional
or supplied gratis by the font.

This also improves the adjustment that takes place for fonts that inject
negative leading whitespace. We now compute the maximum negative leading
whitespace for all lines in multiline text and shift everything over by that
maximum rather than shifting each line over by its negative whitespace.
Presumably the fonts are using negative whitespace for some aesthetic purpose
and this does a better job of preserving it, as much as possible.