Fix #943.

Fix #943.

- Currently, slur direction is calculated once and possibly modified
  later.  Combine these routines into a single function.

- Check for a post-line-break slur first; in this case, the direction
  should come from the pre-line-break slur.

- Check for a pre-line-break slur next; if found, forward its direction
  to the corresponding post-line-break slur.

- If these conditions do not hold, we have an unbroken slur, so simply
  calculate its direction.