Allow for IfcSurfaceStyleShading.Transparency (IFC4+)

[cvillagrasa]

Now, a surface style expressed as a plain IfcSurfaceStyleShading with a Transparency value always renders fully opaque, with GetFlatMesh reporting alpha 1.0. But an IfcSurfaceStyleRendering works:

    #13=IFCCOLOURRGB($,1.,0.,0.);
    #14=IFCSURFACESTYLESHADING(#13,0.8);   -> alpha comes out 1.0, expected 0.2
    #16=IFCSURFACESTYLERENDERING(#13,0.8,$,$,$,$,$,$,.NOTDEFINED.);   -> alpha 0.2, correct

It seems to come from IfcGeometryLoader::GetColor, which reads Transparency only in the IFCSURFACESTYLERENDERING case. The IFCSURFACESTYLESHADING case reads only attribute 0 (SurfaceColour) and recurses into IFCCOLOURRGB, which hardcodes alpha 1. That matches IFC2X3, where IfcSurfaceStyleShading had no Transparency, but in IFC4 and IFC4X3 the attribute was moved up from Rendering to Shading (with Rendering inheriting it).

This PR enables IfcSurfaceStyleShading.Transparency to also have an effect.

tests/functional/ShadingTransparency.spec.ts covers three cases with inline models: IFC4 plain Shading with Transparency 0.8 gives mesh alpha 0.2, IFC4 Rendering with Transparency 0.8 gives alpha 0.2 (passes before and after), and IFC2X3 single-argument Shading stays at alpha 1.0 with no parse error.