Returns the shading model of this material as one of the values of the 
siShadingModel enum. It is usually not possible to exactly 
represent the mental ray shaders attached to a material in OpenGL.  However, Softimage 
tries to guess the best OpenGL shading model to approximate these shaders. Softimage chooses 
a shading model based on specific markings on shader parameters provided by the shader 
developer (using the ui "mapping" = "guid" syntax in the SPDL).  These markings tell 
Softimage if a parameter can be interpreted as a diffuse color, a specular color, etc.  Based 
on the presence or absence of these markings, Softimage guesses a shading model.  For example, 
if there is a diffuse, specular, and ambient-like parameter, Softimage chooses the Phong shading 
model (which in OpenGL, is actually Gouraud).  If only diffuse and ambient-like parameters 
are present, Lambert would be used instead (since the Lambert shading model does not have 
a specular color).  If only ambient is present, Constant is used.