Fix issue 20 for good

[Taiki-San]

Basically, the initial code completely forgot about borders ;p I tried to keep the code properly compartmented, but I quite new at ObjC, so probably far from perfect. However, it works, even if odd values mess stuffs up a bit (but nothing hard to work around), and not on the first time.

Also, for unknown reasons, the patch fails if width get over ~20, but at this point, the bug isn't blocking anymore.