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/render/backend/videorenderworker.h

@@ -42,7 +42,7 @@ protected:
 
   virtual FramePtr RetrieveFromDecoder(DecoderPtr decoder, const TimeRange& range) override;
 
-  virtual NodeValueTable RenderBlock(NodeOutput *output, const TimeRange& range) override;
+  virtual NodeValueTable RenderBlock(TrackOutput *track, const TimeRange& range) override;
 
 private:
   void ProcessNode();