[notodiff] Fairer qualification of render diff
[notodiff] Fairer qualification of render diff

With this change, if two pixels between renderings differ by less than
255 (i.e. one or both is a shade of gray), the pixel difference counts
less towards the overall difference. This should reduce the amount of
noise from glyphs which have changed just a slight amount and whose
rendering difference is barely visible.

googlefonts/nototools,googlefonts/nototools,anthrotype/nototools,moyogo/nototools,anthrotype/nototools,googlei18n/nototools,googlei18n/nototools,googlefonts/nototools,googlefonts/nototools,moyogo/nototools,dougfelt/nototools,googlefonts/nototools,dougfelt/nototools,anthrotype/nototools,dougfelt/nototools,moyogo/nototools,googlei18n/nototools