some render cache adjustments

app/node/processor/renderer/rendererdownloadthread.h

@@ -15,6 +15,9 @@ public:
 
   void Queue(RenderTexturePtr texture, const QString &fn, const rational &time);
 
+public slots:
+  virtual void Cancel() override;
+
 signals:
   void Downloaded(const rational& time);
 
@@ -32,6 +35,8 @@ private:
 
   QMutex texture_queue_lock_;
 
+  QAtomicInt cancelled_;
+
 };
 
 using RendererDownloadThreadPtr = std::shared_ptr<RendererDownloadThread>;