nodes: optimize node graph changes by only updating from inputs that changed

A huge optimization that ensures only the parts of a node graph that have
changed get pushed to the renderer. For thread-safety, the node graph is
copied elsewhere so that users can make changes asynchronously and the graph
can update when its threads are ready. Up until now, if an input value changed,
every node's values would be re-copied, or worse, if a connection was changed,
the entire graph would be recopied. This has been negligible in testing since
we've been largely testing with small graphs, but for massive projects, it's
important that this be as optimized as possible.

app/render/backend/renderbackend.h

@@ -52,11 +52,7 @@ public:
   void CancelQueue();
 
 public slots:
-  void InvalidateCache(const TimeRange &range);
-
-  bool Compile();
-
-  void Decompile();
+  void InvalidateCache(const TimeRange &range, NodeInput *from);
 
 signals:
   void QueueComplete();
@@ -68,10 +64,6 @@ protected:
 
   virtual void CloseInternal();
 
-  virtual bool CompileInternal() = 0;
-
-  virtual void DecompileInternal() = 0;
-
   virtual bool CanRender();
 
   virtual TimeRange PopNextFrameFromQueue();
@@ -111,7 +103,7 @@ protected:
 
   const QString& cache_id() const;
 
-  void QueueValueUpdate();
+  void QueueValueUpdate(NodeInput *from);
 
   bool AllProcessorsAreAvailable() const;
   bool WorkerIsBusy(RenderWorker* worker) const;
@@ -121,18 +113,17 @@ protected:
 
   QVector<RenderWorker*> processors_;
 
-  bool compiled_;
-
   QHash<TimeRange, qint64> render_job_info_;
 
-  QList<Node*> source_node_list_;
+  QHash<Node*, Node*> node_copy_map_;
 
   NodeGraph copied_graph_;
 
-protected slots:
-  void QueueRecompile();
-
 private:
+  void CopyNodeInputValue(NodeInput* input);
+  Node *CopyNodeConnections(Node *src_node);
+  void CopyNodeMakeConnection(NodeInput *src_input, NodeInput *dst_input);
+
   /**
    * @brief Internal list of RenderProcessThreads
    */
@@ -160,8 +151,7 @@ private:
 
   QString cache_id_;
 
-  bool recompile_queued_;
-  bool input_update_queued_;
+  QList<NodeInput*> input_update_queued_;
 
   QVector<bool> processor_busy_state_;