Add check to ensure a base glyph isn't classified as a mark glyph in GDEF table

We pushed Hebo VF to production yesterday and a user reported that the comma and underscore are not working as intended, https://github.com/google/fonts/issues/2453.

When I inspect the GDEF table, we can see that the “comma” is a class 3 aka a “Mark” glyph, https://docs.microsoft.com/en-us/typography/opentype/spec/gdef#glyph-class-definition-table

(venv) Marcs-Air:heebo marcfoley$ grep "comma" Heebo\[wght\].ttx
      <ClassDef glyph="comma" class="3"/>

When I inspect the source files, we can see a mark anchor exists in the comma.

An _ denotes that this glyph is a mark glyph

To ensure that base glyphs are not classified as a mark glyph, we could raise a fail If a glyph has an advance width greater than 0 and it exists in the gdef.GlyphClassDef as class 3.

More than happy to submit a pr.

[anthrotype]

we could raise a fail If a glyph has an advance width greater than 0 and it exists in the gdef.GlyphClassDef as class 3

yes, that could be a signal that something is not ok, either the GDEF classifies a base glyph incorrectly as a mark, or a glyph that is supposed to be used as a mark has an incorrect advance width that should have been 0.

Note that in glyphsLib we automatically set the advance width to 0 for glyphs that are classified as nonspacing marks (via GlyphData.xml or custom glyph properties).

Technically one does not need to set width to 0 because OT layout engines would automatically do when if a glyph is classified as mark.

[anthrotype]

IMO it would be better to recommend that the sources contain an explicit GDEF table definition in the UFO/features.fea (this is created automatically by fontmake for .glyphs sources using GlyphData.xml database in glyphsLib). When there is an explicit GDEF definition in the features.fea, ufo2ft does not guesses whether a glyph is a mark by the presence of an _-prefixed anchor, but instead it uses the GDEF to classify glyphs as bases, marks and ligatures accordingly.