Lots of improvements to the path flow.

- Interpolation is now done at the motion level
  as opposed to the higher path lever
  so an arcCW when not supported by the driver
  can be interpolated into line segments.

- Arcs are detected in offsetting and their radii
  is offset so they can be preserved all the way
  to the driver in offsetted fills and strokes.

- Cleaned up the way helical plunging tracks distance.