Slur between voices too curved

1) default empty score
2) voice 1: 4 quarter notes, top line F
3) voice 2: 4 quarter notes, bottom space F
4) slur first to last high F
5) flip slur below

Result:

There are other ways to create similar situations. Basic idea is, the slur actual would have fit between the notes without adjustment, but as of my recent change to curve slurs more to avoid collisions, we don't take advantage of that. Simple fix, PR to follow.