[notodiff] Fix shape ratio calculation
[notodiff] Fix shape ratio calculation

Before, this didn't account for negative areas.

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