removed all dependence on NodeOutputs

If Nodes only have the one output, we don't need to do so much differentiation
between them. Previous iteration used outputs as like a distinct function
within a Node (e.g. length output would return one result, buffer output would
produce a different result - each run different code to produce their results).
Now in this iteration, it's more accurate to say a Node is just one function
(which seems more appropriate for a node system anyway).

app/node/generator/solid/solid.h

@@ -41,15 +41,11 @@ public:
   virtual QString Category() const override;
   virtual QString Description() const override;
 
-  NodeOutput* texture_output() const;
-
-  virtual QString Code(NodeOutput* output) const override;
+  virtual QString Code() const override;
 
 private:
   NodeInput* color_input_;
 
-  NodeOutput* texture_output_;
-
   QOpenGLTexture* texture_;
 };