[Fix] Compute the specular color using the IfcSurfaceStyleRendering specular color/factor instead of using the diffuse color.
[Fea] Arbitrary choice: don't set the ambiant color as 80% of the diffuse color but to 0.2, 0.2, 0.2 (dark grey).
[Fea] Lighting model using a more meaningful DIFFUSE mode instead of SPECULAR.