Fix node_label() function to work with self-intersection

Fix node_label() function to work with self-intersection

Rather than just giving up if we encounter our own edges in the
CVC list at first, skip them until we either run out of edges, or
find one belonging to the other polygon.

I'm not 100% sure this is the correct fix, but it "seems to work".

Test-case:

Layer(1 "component")
(
  Line[60000 70000 60000 90000 4000 2000 "clearline"]
  Line[80000 60000 80000 90000 4000 2000 "clearline"]
  Line[90000 90000 90000 50000 4000 6000 "clearline"]
  Line[60000 40000 80000 60000 4000 6000 "clearline"]
  Polygon("clearpoly")
  (
    [10000 10000] [140000 10000] [140000 140000] [10000 140000]
  )
)