Got distracted from going to bed by the desire to have 4-point coplanar contours properly tessellate. Wound up having to write a new utility function to determine whether a cloud of points is coplanar.

Not a particularly involved function, just note that for all sets > 3 points, you create a vector from A to B, and cross it with set of vectors from C to N. Eliminate all NULL vectors (those points colinear with A and B), then normalise all remaining vectors. If all vectors are equal, points are coplanar, otherwise they are not.

About time I got to bed. But for the curious, here's what that "coplanar?" test looks like in code: