Fix rendered characters have been chipped for some TrueType fonts

ImageFont ignores descender value of TrueType fonts (uses ascender only),
then some fonts which use descender is chipped on rendering.
![chipped_descenders-ipafont-gothic](https://f.cloud.github.com/assets/748828/113674/eee8934c-6b6c-11e2-8ffd-d3525ecc90c2.png)

This fix is already reported to PIL at http://hg.effbot.org/pil-2009-raclette/issue/13/chipped-characters-have-been-rendered