message: Give access to class derived from SurfaceNodeGLSL
  One such class is the SurfaceNodeMaya as used by MayaUSD.
diff: @@ -25,7 +25,7 @@ class MX_GENGLSL_API SurfaceNodeGlsl : public GlslImplementation
  virtual void emitLightLoop(const ShaderNode& node, GenContext& context, ShaderStage& stage, const string& outColor) const;
  - private:
  + protected:
  /// Closure contexts for calling closure functions.
  mutable ClosureContext _callReflection;
  mutable ClosureContext _callTransmission;
  