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

Use the alpha channel, not the red channel. Fixes commit f92830.

Original patch by lachs0r.

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