2 sided faces aren't supported and will cause problems in many areas
of Blender's code.
Removing (implied) support for faces with fewer than 3 sides
means the total number of triangles is known ahead of time.
This simplifies adding support for multi-threading and partial updates
to an existing tessellation - as the face and loop indices can be used
to access the range of triangles associated with a face.
Also correct outdated comments.