New check: consistent underline position and thickness

[dscorbett]

What needs to be checked?

underlinePosition and underlineThickness in the 'post' table should be consistent with UnderlinePosition and UnderlineThickness in the 'CFF ' table.

Detailed description of the problem

The specification of underlinePosition in 'post' says “The value of the PostScript key may be calculated by subtracting half the underlineThickness from the value of this field.” That is, CFF.UnderlinePosition == post.underlinePosition - post.underlineThickness / 2 by definition.

underlineThickness in 'post' and UnderlineThickness in 'CFF ' are both simply the underline thickness, so they should always equal each other.

Suggested profile

Suggest which profile the check should be added to. The most common are:

• [ ] Vendor-specific: Google Fonts
• [ ] Vendor-specific: Adobe Fonts
• [x] OpenType (requirements imposed by the OpenType specification)
• [x] Universal (broadly accepted best practices on the type design community)
• [ ] Other:

Suggested result

Which log result level should the check have:

• [ ] 🔥 FAIL (An issue that must be corrected for the font to function properly)
• [x] ⚠️ WARN (A potential issues that may need to be addressed)

Severity assessment

1 (minor)

[benkiel]

See https://github.com/googlefonts/ufo2ft/pull/764 and https://github.com/unified-font-object/ufo-spec/pull/221 which will make this easier to deal with in sources