VSFilter compat: don't render shadow when glyph/border invisible
VSFilter compat: don't render shadow when glyph/border invisible

Yet another VSFilter idiosyncracy: when the glyph fill is invisible
(alpha 0xFF) and there is no border, do not render any shadow.

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