cache: better implemented changing disk cache folders

Not complete yet, but getting there.

app/render/backend/renderworker.h

@@ -48,6 +48,11 @@ public:
     available_ = a;
   }
 
+  void SetViewerNode(ViewerOutput* viewer)
+  {
+    viewer_ = viewer;
+  }
+
   void SetVideoParams(const VideoParams& params)
   {
     video_params_ = params;
@@ -171,6 +176,8 @@ private:
 
   bool generate_audio_previews_;
 
+  ViewerOutput* viewer_;
+
   QHash<Node*, Node*>* copy_map_;
 
   RenderMode::Mode render_mode_;