moved frame cache saving to our custom thread pool so we can control the priority

app/render/rendermanager.h

@@ -91,11 +91,14 @@ public:
    */
   RenderTicketPtr RenderAudio(ViewerOutput* viewer, const TimeRange& r, bool prioritize = false);
 
+  RenderTicketPtr SaveFrameToCache(FrameHashCache* cache, FramePtr frame, const QByteArray& hash, bool prioritize = false);
+
   virtual void RunTicket(RenderTicketPtr ticket) const override;
 
   enum TicketType {
     kTypeVideo,
-    kTypeAudio
+    kTypeAudio,
+    kTypeVideoDownload
   };
 
 signals:
@@ -105,6 +108,8 @@ private:
 
   static void RenderAudioInternal(RenderTicketPtr ticket);
 
+  static void SaveFrameToCacheInternal(RenderTicketPtr ticket);
+
   RenderManager(QObject* parent = nullptr);
 
   virtual ~RenderManager() override;