renderer: use a dedicated decoder per thread

app/render/backend/audio/audiobackend.cpp

@@ -46,7 +46,7 @@ bool AudioBackend::InitInternal()
   // Initiate one thread per CPU core
   for (int i=0;i<threads().size();i++) {
     // Create one processor object for each thread
-    AudioWorker* processor = new AudioWorker(decoder_cache(), &copy_map_);
+    AudioWorker* processor = new AudioWorker(&copy_map_);
     processor->SetParameters(params());
     processors_.append(processor);
   }