flat primitives scaling

[Philippe Hurbain]

I suggest to update text from primitive reference from

To avoid matrix arithmetic problems in some renderers, the third dimension ({y} in the default orientation) of two-dimensional primitives must be given a scaling factor of 1.

to

To avoid matrix arithmetic problems in some renderers, the third dimension ({y} in the default orientation) of two-dimensional primitives must be given a non-zero scaling factor.

There is no reason to restrict this value to 1, the only point is to avoid a non-degenerated transformation matrix.
This change should avoid sterile discussions such as this one: http://www.ldraw.org/cgi-bin/ptdetail.cg.../u9228.dat