s = vrml_flatten (x [, d, w, col]) - A planar surface containing x

If the points  x  are not coplanar (or not in the affine plane {y|d'*y==w}),
the surface will not contain the points, but rather their projections on
the plane {y|d'*y==w}.

x   : 3 x P  : 3D points
d   : 3      : normal to plane    | Default : given by best_dir()
w   : 1      : intercept of plane |
col : 3      : RGB color            Default : [0.3,0.4,0.9]

s   : string : vrml code representing the planar surface