previewautocacher: ensure queues are actually cleared

app/render/previewautocacher.cpp

@@ -378,6 +378,7 @@ void PreviewAutoCacher::ClearHashQueue(bool wait)
       (*it)->waitForFinished();
     }
   }
+  hash_tasks_.clear();
 }
 
 void PreviewAutoCacher::ClearVideoQueue(bool wait)
@@ -395,6 +396,7 @@ void PreviewAutoCacher::ClearVideoQueue(bool wait)
     }
   }
 
+  video_tasks_.clear();
   has_changed_ = true;
   use_custom_range_ = false;
 }
@@ -413,6 +415,7 @@ void PreviewAutoCacher::ClearAudioQueue(bool wait)
       it.key()->WaitForFinished();
     }
   }
+  audio_tasks_.clear();
 }
 
 void PreviewAutoCacher::ClearVideoDownloadQueue(bool wait)
@@ -429,6 +432,7 @@ void PreviewAutoCacher::ClearVideoDownloadQueue(bool wait)
       it.key()->WaitForFinished();
     }
   }
+  video_download_tasks_.clear();
 }
 
 void PreviewAutoCacher::NodeAdded(Node *node)