Blender Git Loki

BMesh: remove checks for tessellating 2 sided faces

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Also correct outdated comments.