Make sure to release each event's vector clip mask
Make sure to release each event's vector clip mask

Events with a clip mask but no text such as just
    Dialogue: ...,{\clip(m 47 32 )}
had their clip masks carried over to the next event.

Fixes GitHub issue #22.

astiob/libass,stps/libass,libass/libass,Akemi/libass,ShiftMediaProject/libass,mwgoldsmith/libass,Akemi/libass,pigoz/libass-ct,mwgoldsmith/libass,ShiftMediaProject/libass,libass/libass,MrSmile/libass,MrSmile/libass,sanyaade-g2g-repos/libass,pigoz/libass-ct,grigorig/libass,sanyaade-g2g-repos/libass,mwgoldsmith/libass,torque/libass,Akemi/libass,stps/libass,torque/libass,torque/libass,astiob/libass,sanyaade-g2g-repos/libass,xbmc/libass,xbmc/libass,grigorig/libass,pigoz/libass-ct,stps/libass