- Fix brainfade in previous delta
- Move ellipsification results to static memory; they are going to
  have to be referenced inside NextLineW and the arg list is too big
  already.
- Add the missing ellipsification result (len_under) to simplify coding.
- Slight correction to prefix tracking for when there are several
  underlined characters on a single line (DT_EXPANDTABS only).
- Allocate and free the temporary copy required for DT_MODIFYSTRING
  rather than using a fixed size stack array.
- Introduce lastline into the main loop; it simplifies the code and we
  are going to need it.