Trace option: break trace for distant verts

[zippy731]

Due to the sequence of vert selection, some traces have long drags across the object because the local verts are exhausted.

Set up a distance limit x such that if next vert distance > x, trace would start a new spline at the next vert. Alt: make a new spline starting at the most distant vert, to generate 'regions of activity' during trace