rem: font_slant(const Cff::Font *family_cff)
add: font_slant(const FontInfo &finfo)
context: font_slant(const Cff::Font *family_cff){    double val;    (void) family_cff->dict_value(Efont::Cff::oItalicAngle, &val);    return -tan(val * M_PI / 180.0) + slant;}