renderer: use a dedicated decoder per thread

app/render/backend/renderworker.h

@@ -35,7 +35,7 @@ class RenderWorker : public QObject, public NodeTraverser
 {
   Q_OBJECT
 public:
-  RenderWorker(DecoderCache* decoder_cache, QObject* parent = nullptr);
+  RenderWorker(QObject* parent = nullptr);
 
   bool Init();
 
@@ -77,7 +77,7 @@ protected:
 private:
   bool started_;
 
-  DecoderCache* decoder_cache_;
+  DecoderCache decoder_cache_;
 
   NodeDependency path_;