shaper: skip invisible characters earlier
shaper: skip invisible characters earlier

Currently, this move does not affect functionality, as both shapers
ignore `skip` on input, and the complex shaper resets `skip` on output.
Later commits will use this `skip` flag to guide shaping run splitting
and font fallback before shaping happens.

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