Harfbuzz ssty feature

buf = hb_buffer_create();
hb_buffer_set_script(buf, HB_SCRIPT_LATIN);
hb_buffer_set_direction(buf, HB_DIRECTION_LTR);
hb_buffer_set_language(buf, hb_language_get_default());

hb_feature_t *features = NULL;
const hb_tag_t math_ssty_tag = HB_TAG('s', 's', 't', 'y');
int num_features = 0;
if (cur_style <= math_script_style) {
    features = malloc(sizeof(hb_feature_t));
    num_features = 1;
    features[0] = (hb_feature_t){ math_ssty_tag, 1, 0, UINT_MAX };
}

hb_buffer_add_utf32(buf, unicode_char, 1, 0, 1);
hb_shape(font, buf, features, num_features);

glyph_info   = hb_buffer_get_glyph_infos(buf, &glyph_count);

assert(glyph_count == 1);

hb_codepoint_t glyph_code = glyph_info[0].codepoint;