Before sending each four-sided body polygon of a sweep
to the output, sketch tests to see if it is roughly planar.
Since planarity is necessary for proper functioning of the hidden
surface algorithm, “warped” polygons are automatically split into
two triangles.
Hole-filling polygons produced by closure-tagged line sweeps are not split. Nor are original polygons in polygon sweeps. It is the user's responsibility to ensure these are planar.