Issues to fix before TTFs are production-ready

[Lorp]

This issue tracks what needs to be fixed in exported static TTFs before they can be used for production. Fixed subissues are marked as checked. New subissues may be added to the list.

• glyph bounding boxes (simple glyphs)
• glyph bounding boxes (composite glyphs)
• head.modified date
• table checksums
• GSUB table (includes rvrn feature variations)
• GPOS table (instantiate kerning)
• MVAR table (instantiate OS/2 table and other font-wide metrics)
• cvar table (instantiatecvt table, turns variable hinting into static hinting)
• font bounding box (in head table)

Note: The static TTF instances generated by Samsa-GUI and Samsa-CLI both use the SamsaFont.exportInstance() method in samsa-core.js, so binaries are identical.