Add support for \fs+ and \fs- syntax
Add support for \fs+ and \fs- syntax

These forms can be used to add or subtract a value from the
current font size instead of setting a new font size. Animations
are supported, but not recommended, as they won't be fluid due to
grid-fitting/hinting.

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