hold worker busy state in renderbackend rather than in renderworker

Workers run in different threads and the backends can poll whether the worker
is currently busy or not. However the previous iteration has the worker (and an
atomic int) provide the busy state which could easily desync with the main
thread (since all workers run in different threads). By holding the busy states
in the main thread, the main thread will always be able to poll the busy state
accurately.

app/render/backend/renderworker.h

@@ -20,8 +20,6 @@ public:
 
   bool IsStarted();
 
-  bool IsAvailable();
-
 public slots:
   void Close();
 
@@ -30,8 +28,6 @@ public slots:
   NodeValueTable RenderAsSibling(NodeDependency dep);
 
 signals:
-  void RequestSibling(NodeDependency path);
-
   void CompletedCache(NodeDependency dep, NodeValueTable data);
 
 protected:
@@ -56,8 +52,6 @@ protected:
 
   virtual NodeValueTable RenderBlock(TrackOutput *track, const TimeRange& range) = 0;
 
-  QAtomicInt working_;
-
 private:
   bool started_;