renderer: share render backend for auto-cache tasks

Since auto-cache events start and stop fairly frequently (and are somewhat
heavy to create/destroy), we can save a lot of cycles over time by sharing
the same backend between them all.

app/render/backend/renderbackend.h

@@ -43,6 +43,11 @@ public:
 
   void Close();
 
+  ViewerOutput* GetViewerNode() const
+  {
+    return viewer_node_;
+  }
+
   void SetViewerNode(ViewerOutput* viewer_node);
 
   void SetUpdateWithGraph(bool e)
@@ -81,6 +86,16 @@ public:
    */
   RenderTicketPtr RenderAudio(const TimeRange& r);
 
+  const VideoParams& GetVideoParams() const
+  {
+    return video_params_;
+  }
+
+  const AudioParams& GetAudioParams() const
+  {
+    return audio_params_;
+  }
+
   void SetVideoParams(const VideoParams& params);
 
   void SetAudioParams(const AudioParams& params);