Refactored RendererServices::RenderState.

Moved it out of RendererServices so it is no longer a nested class. Added public userData() method so that RendererServices no longer needs private access. This is in preparation for using the RenderState class to implement most of the work involved in managing shading state.
Refactored OSL ShadingEngine::shade to use new ShadingResults class.

This is in preparation for adding a debug closure type which will enable the creation of multiple outputs.
Added debug closure support to OSLRenderer.