fixing shader viewer

The current viewer node is just and emission shader, which is perfectly fine, but this way it's visible to every other object, ending up making the material emissive. Not a big deal, but it's an even smaller deal to make it a group with a camera ray plugged into its emission (since you are not supposed to alter the brightness of the viewer anyway). Seems like nothing, but those who work a lot with procedural shading would appreciate this a lot (and it would be an improvement for everyone anyway).