fix: ON_Material::MaxShine is not a function

[aminya]

Cherry-picked from #28

• It fixes a bug in opennurbs_gl where ON_Material::MaxShine which is a constant was being called as a function.

C:/opennurbs/opennurbs_gl.cpp:603:71: error: called object type 'double' is not a function or function pointer
    GLint shine = (GLint)(128.0*(pMat->Shine() / ON_Material::MaxShine()));
                                                 ~~~~~~~~~~~~~~~~~~~~~^

[sbaer]

We don't actually compile this legacy source file any of our projects which is why it went by unnoticed. I suspect the CMake PR will need to be adjusted as this was probably discovered by compiling all source files in the opennurbs directory.