[Gs-code-review] Fix fuzzy.c for strange depths

[Jack Moffitt]

> This patch produces more meaningful results when applied to pgm and
> ppm files of depths other than the usual 2^bps - 1. This is important
> because the pkmraw driver in Ghostscript produces ppm files of depth
> 1.
> 
> I'm happy to have either Jack or Ray review this. Jack is probably the
> best person to test it, because the issue came up while working on
> pdfwrite regression.

I have confirmed that this patch fixes the problem.  Please commit.

jack.