When building a variable font, varLib.build must make sure that
the OS/2.usWeightClass is equal to the wght axis default location,
that the OS/2.usWidthClass is set to the equivalent value (1-9) of
the wdth axis default location, and finally that post.italicAngle
is set to the same default value as slnt axis.
Sometimes the base master doesn't have these values correctly
set leading to discrepancies between OS/2 and post, on the one
hand, and the fvar axes' default values.
