 LIGHT property for surfaces with lighting
 
    Use the property/value pair 'light',V in the argument
    list of surfl or trisurfl to define the direction of
    light. The 3-Vector V=[x,y,z] or the 2-vector V=[az,el]
    give the normalized coordinates of the light source at
    infinity.
    
    Examples :
    
    deff('z=f(x,y)','z=x.^2-y.^2');    
    surfl(x,y,f,'shading','flat','light',[0 0 1])
    
