Font metrics: prefer OS/2 table for ascender/descender
Font metrics: prefer OS/2 table for ascender/descender

If possible, use ascender/descender from the OS/2 TTF table. This is
another try to get libass to match VSFilter in regard to vertical font
metrics.

rhlee/libass,pigoz/libass-ct,MrSmile/libass,tgoyne/libass,richardpl/libass,pigoz/libass-ct,Distrotech/libass,sanyaade-g2g-repos/libass,stps/libass,sanyaade-g2g-repos/libass,stps/libass,asimonov-im/libass,blinkseb/libass,tgoyne/libass,Distrotech/libass,richardpl/libass,pigoz/libass-ct,stps/libass,tgoyne/libass,sanyaade-g2g-repos/libass,torque/libass,torque/libass,vault/libass,libass/libass,vault/libass,libass/libass,ShiftMediaProject/libass,MrSmile/libass,Akemi/libass,Akemi/libass,astiob/libass,blinkseb/libass,xbmc/libass,asimonov-im/libass,astiob/libass,grigorig/libass,mwgoldsmith/libass,Distrotech/libass,rhlee/libass,blinkseb/libass,mwgoldsmith/libass,mwgoldsmith/libass,ShiftMediaProject/libass,rhlee/libass,grigorig/libass,Akemi/libass,xbmc/libass,richardpl/libass,torque/libass