subsurface_color needs to be clamped to 0->1

[BrianSharpe]

The OpenPBR spec states that subsurface_color should be in the range of 0 to 1, but this is not enforced by the node graph.

This has caused an issue with the OpenPBR Shader Playground scene, where the paper surface on which the toy plane sits has a subsurface_color input in the range of ~3.0 to 4.0 (coming in through a wire).

[BrianSharpe]

closing, was superceded by #276