mkhexgrid TODO list:

* Grid thickness must be 1px with antialiased PNG output due to a limitation
  in libgd, which we use for PNG drawing. In order to fix this, we need
  either to write a custom routine for drawing thick antialiased lines or
  get such a routine added to libgd.

* It would be nice if common paper sizes (letter, legal, A4, D) could be
  given for the image size for PostScript output.

* Add a center thickness option for cross centers.

* PDF output.

* Do some other check for GDFONTPATH on Windows. Fonts are not always in
  C:\Windows\Fonts.

* Include tutorial in package. Expand tutorial.

* Include Japanese translation of manual in package.