VF designspaces need public.skipExportGlyphs key set to match GOADB

[punchcutter]

Without the key set the TTFs will contain more glyphs which causes problems with the font in ots.

[punchcutter]

To be more clear about this issue the current build process shares some tables from the OTF CFF2 build with the variable TTF, but not all tables. If there are more glyphs in a font than specified in the GlyphOrderAndAliasDB file makeotf will drop them from the resulting font so we end up with an OTF that has fewer glyphs than the original source. The variable TTF built by fontmake will contain all of the source glyphs unless we specify public.skipExportGlyphs. If we don't then the TTF will end up with more glyphs than the OTF which by itself doesn't hurt anything. The buildVFs.sh script, however, copies the hhea from the OTF so if the number of glyphs differs we have a conflict. The hhea.numberLongMetrics field is the number of metrics in the hmtx table. Without dropping the glyphs in the TTF we end up with a mismatch. ttx spits out the message "WARNING: too much 'hmtx' table data" as reported at adobe-fonts/source-serif#110