ptem, used for AAT's tracking/`trak` table is equivalent to opsz of variable fonts.
For variable AAT fonts, such as SFNS, ideally variable axis of the hb_font_t
should be set and equivalent to ptem, https://crbug.com/1005969#c37
Searches variation axes of a hb_font_t object for a specific axis first,
if not set, then tries to get default style values from different
tables of the font.
Khaled Hosny
Ebrahim Byagowi