Triangles and quads don't check for degenerate faces (where the normal
might be zero), while ngons already set the Z component to 1 for that,
which is also what we do for vertex normals.
Also reorder the face size checks to put quads and triangles before
N-gons, since they are typically more common on high-poly meshes.
And change the check for 2 sided faces into an assert.
f557222a7b