[varLib instancer] should set up italic style linking as an option

When slicing a VF with ital or slant axis, varLib instancer could set up proper italic style bits + italic angle + caret slope if italic != 0 and if slant != 0. For example with and additional argument like --is-italic followed by an integer for italic angle.

For example when running: fonttools varLib.instancer "../fonts/variable/Nunito[ital,wght].ttf" ital=1 wght=200:1000 -o "../fonts/variable/Nunito-Italic[wght].ttf"

Then we have to patch this kind of metadata:

for font_path in glob.glob("../fonts/variable/*Italic*.ttf"):
        with open(font_path, "rb") as f:
            print("Fix italic style linking in {}".format(font_path))
            ttFont = TTFont(f)
            ttFont["head"].macStyle = MacStyle.ITALIC
            ttFont["post"].italicAngle = -9
            ttFont["hhea"].caretSlopeRise = 1000
            ttFont["hhea"].caretSlopeRun = 158
            ttFont.save(font_path)

[RosaWagner]

Is this documented anywhere, and is there a fontbakery check for it?

There are several links and conversation in this issue: https://github.com/googlefonts/gftools/issues/297. Our last testing about this is from this summer 2022, didn’t check if CC23 has fixed it yet. Fontbakery’s related issue is still opened.

[RosaWagner]

Hi, sorry I was on holidays.

The PR did fixed the style linking issue but I still see 2 things that would influence user experience:

• italic angle is still 0
• caretSlopeRise and caretSlopeRun are still 1 and 0 respectively. They can be calculated from the italic angle.