Fix the bug about the direction of Normal.

[nothinglo]

I have tested. It seems wrong direction of the triangle normal.

[DaviesX]

If the order of those 3 vertices is counter-clockwise, then the normal is indeed v_12 x v_13.

[mmp]

Changing this would end up flipping the normals of all triangles, which in turn would cause images to be quite different. Admittedly we haven't documented the expected vertex ordering, but given @DaviesX 's observation, I think that decreeing that they are counter-clockwise and leaving the code as is is the best thing at this point.