Robust sections on Mesh objects

When dealing with a badly defined mesh (overlapping triangles, edges shared by more than two triangles and so on) our Mesh.Section() method can fail.

The following code shows how to extract a section even in these problematic situations.

Mesh sectioning methods can be used also for surfaces, after converting a surface to a mesh by Surface.ConvertToMesh().

Mesh mesh = Mesh.CreateTorus(5, 2, 10, 20); 
mesh.Translate(110, 30, 0); 
singleViewportLayout1.Entities.Add(mesh, 0, Color.ForestGreen); 

Plane pln = Plane.YZ; 

pln.Origin = new Point3D(108, 0, 0); 

foreach (IndexTriangle it in mesh.Triangles) 
    Point3D p1 = mesh.Vertices[it.V1]; 
    Point3D p2 = mesh.Vertices[it.V2]; 
    Point3D p3 = mesh.Vertices[it.V3]; 

    Segment3D s1 = new Segment3D(p1, p2); 
    Segment3D s2 = new Segment3D(p2, p3); 
    Segment3D s3 = new Segment3D(p3, p1); 

    Point3D intPoint1; 
    bool rc1 = s1.IntersectWith(pln, out intPoint1); 

    Point3D intPoint2; 
    bool rc2 = s2.IntersectWith(pln, out intPoint2); 

    Point3D intPoint3; 
    bool rc3 = s3.IntersectWith(pln, out intPoint3); 

    Line result = null; 

    if (rc1 && rc2) 
        result = new Line(intPoint1, intPoint2); 

    else if (rc2 && rc3) 
        result = new Line(intPoint2, intPoint3); 

    else if (rc3 && rc1) 
        result = new Line(intPoint3, intPoint1); 

    if (result != null && result.Length() > 1e-3) 
        singleViewportLayout1.Entities.Add(result, 0, Color.Black); 
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