emission_luminance needs to be clamped to >0

[BrianSharpe]

(similar to this)

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

This has caused an issue with the OpenPBR Shader Playground scene, where the meetMat model has negative values coming in from a wire.

This issue is mentioned in the OpenPBR Shader Playground repro too.
DigitalProductionExampleLibrary/OpenPBRShaderPlayground#5

[BrianSharpe]

closing, was superceded by #276