fix_weight_class variable font improvement

[ollimeier]

According to Microsoft's OT spec the usWeightClass should match the fvar default value.
In most of the cases this might be 400, but in some it's not.

fix_weight_class does not take care of that:

gftools/Lib/gftools/fix.py

Line 184 in f0d02b3

def fix_weight_class(ttFont):

The OT Spec says:
In a variable font that implements 'wght' variations, the value in the usWeightClass field of the OS/2 table must match the default 'wght' value specified in the 'fvar' table.
https://docs.microsoft.com/en-us/typography/opentype/spec/dvaraxistag_wght#additional-information

That do you think about something like that:

def fix_weight_class_var(font_obj):
    if 'fvar' not in font_obj:
        # not a variable font
        return

    fvar = font_obj['fvar']
    default_axis_values = {a.axisTag: a.defaultValue for a in fvar.axes}

    print('default_axis_values: ', default_axis_values)
    v = default_axis_values.get('wght', None)

    if isinstance(v, int) or isinstance(v, float):
        font_obj["OS/2"].usWeightClass = int(v)
        print('Changed usWeightClass tom match fvar default value.')

[simoncozens]

This sounds like a good idea. We should also have a fontbakery check which does this.

By the way, the second if test could just be if v is not None.

[ollimeier]

@simoncozens I just made a PR :) I will also add that to fontbakery.

[m4rc1e]

Fixed in #481