A function that is called with pairs of crossing edges. The function may return false in order to request that the algorithm should be terminated, i.e. no further crossings are needed.
Visits all pairs of crossing edges in the given S2ShapeIndex, terminating early if the given EdgePairVisitor function returns false (in which case VisitCrossings returns false as well). "type" indicates whether all crossings should be visited, or only interior crossings.
Like the above, but visits all pairs of crossing edges where one edge comes from each S2ShapeIndex.