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.