Fix issue 3868: line-breaking makes short lines

Fix issue 3868: line-breaking makes short lines

Two simple fixes:
1) Always have some stretchability between notes
2) If there _is_ a completely unstretchable line, shorter than the line-width,
   score it as a bad fit, not as a perfect fit (report large forces for stiff lines).

Either one would fix the problem; both seem to be good to have in the code.