Ensure Opacity default is 1.0

Introduce an extended PBR-style material. The extended material
hosts now smudge and scratch (bump) maps.

* Subsurface Scattering Radius as float instead of color

* Opacity IOR to default 1.45 from 1.0
* Opacity default at 1.0 instead of 0.0

+v6.16

Remove separate normal channel.

Rhino provides an API to determine whether
an image is a bump or normal map.

Also remove the displacement entry, since it isn't
likely going to be part of the material
defintion

Make sure the plug-in does not depend on RhinoCycles-specific code.