renderer: use shared backend when caching

I think there was an earlier commit with a similar name but turns out
I'd only done foundational work in that commit and never actually
properly set it up. Of course once I did, there were several issues that
needed fixing to make it work correctly, but now it works as expected.

Heavily optimizes larger projects by allowing cache jobs to only copy
what has changed.

app/render/backend/renderbackend.h

@@ -69,12 +69,10 @@ public:
 
   void ProcessUpdateQueue();
 
-  static QByteArray HashNode(const Node* n, const VideoParams& params, const rational& time);
-
   /**
    * @brief Asynchronously generate a hash at a given time
    */
-  QFuture<QVector<QByteArray> > Hash(const QVector<rational> &times);
+  RenderTicketPtr Hash(const QVector<rational> &times);
 
   /**
    * @brief Asynchronously generate a frame at a given time