Stop hard-coding a nonstandard font. We now default to `fixed', and

pick up the font's real width and height. This means I now _can't_
use my font of choice until I implement some command-line options; I
wonder what feature will appear next :-)

git-svn-id: svn://svn.tartarus.org/sgt/putty@2027 cda61777-01e9-0310-a592-d414129be87e