copy values whenever the cache is invalidated

Since we're now working with a separate proxy copy of the original node graph,
if the user changes a parameter in one of those nodes (triggering an
InvalidateCache signal), the values in our copied graph need to be updated
with these new values too.

app/render/backend/renderbackend.h

@@ -85,6 +85,10 @@ protected:
 
   const QString& cache_id() const;
 
+  void QueueValueUpdate(const TimeRange& range);
+
+  void UpdateNodeInputs();
+
   QList<TimeRange> cache_queue_;
 
   QVector<RenderWorker*> processors_;
@@ -125,8 +129,12 @@ private:
   qint64 cache_time_;
   QString cache_id_;
 
+  QList<Node*> source_node_list_;
   NodeGraph copied_graph_;
 
+  bool value_update_queued_;
+  TimeRange value_update_range_;
+
   bool recompile_queued_;
 
 private slots: