stroker: correctly handle case of many small segments
stroker: correctly handle case of many small segments

Fixes: https://github.com/libass/libass/issues/326.

libass/libass,MrSmile/libass,Akemi/libass,ShiftMediaProject/libass,xbmc/libass,Akemi/libass,astiob/libass,Akemi/libass,xbmc/libass,astiob/libass,ShiftMediaProject/libass,MrSmile/libass,libass/libass