neatogen mkOverlapSet: return actual point count in 'cntp'

neatogen mkOverlapSet: return actual point count in 'cntp'

For reasons unclear to me, the `mkOverlapSet` function was constructing a set of
points with an implicit initial 0 point and then notifying the caller of a count
1 _less_ than the total point count. There was nothing wrong with this, but it
led to several instances of non-idiomatic follow-on code that had to account for
an array that was actually 1 longer than what its count variable said.