overlap method
This function can be overridden if the original chains are needed
@param start1 the index of the start of the overlapping segment from mc1 @param start2 the index of the start of the overlapping segment from mc2
Implementation
void overlap(MonotoneChainI mc1, int start1, MonotoneChainI mc2, int start2) {
mc1.getLineSegment(start1, overlapSeg1);
mc2.getLineSegment(start2, overlapSeg2);
overlapLS(overlapSeg1, overlapSeg2);
}