HelixToolkit.Wpf.SharpDX: Fix ArgumentOutOfRangeException in MeshBuilder.ComputeNormals.

Current ComputeNormals method tries to access an item of zero-length list. The capacity property is initialized, but the Count is 0.

    private static void ComputeNormals(Vector3Collection positions, IntCollection triangleIndices, out Vector3Collection normals)
    {
        normals = new Vector3Collection(positions.Count);

        for (int t = 0; t < triangleIndices.Count; t += 3)
        {
            // ...

            normals[i1] += (a * n); // ArgumentOutOfRangeException
            normals[i2] += (a * n);
            normals[i3] += (a * n);
        }
        
        // ...