viewer: receive audio invalidation signal

Invalidates and runs cache task correctly even if there are no video
nodes connected.

app/task/render/render.cpp

@@ -233,6 +233,7 @@ void RenderTask::Render(const TimeRangeList& video_range,
     }
   }
 
+  // `Close` will block until all jobs are done making a safe deletion
   backend_.Close();
 }