[notodiff] Don't render if one glyph is zero-width
[notodiff] Don't render if one glyph is zero-width

If one glyph in a comparison will be rendered with width zero (either
because its width is zero or because it has class mark), the rendered
comparison is not very helpful. This prints an error message instead.

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