public void Join(ContourPolygon pol)
{
var size = NumberOfContours;
Contours.AddRange(pol.Contours);
foreach (var contour in pol.Contours)
{
contour.ClearHoles();
foreach (var holeId in contour.Holes)
{
Contours.Last().AddHole(holeId + size);
}
}
}