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/renderticket.h

@@ -35,13 +35,14 @@ class RenderTicket : public QObject
   Q_OBJECT
 public:
   enum Type {
+    kTypeHash,
     kTypeVideo,
     kTypeAudio
   };
 
-  RenderTicket(Type type, const TimeRange& time);
+  RenderTicket(Type type, const QVariant& time);
 
-  const TimeRange& GetTime() const
+  const QVariant& GetTime() const
   {
     return time_;
   }
@@ -82,7 +83,7 @@ private:
 
   QWaitCondition wait_;
 
-  TimeRange time_;
+  QVariant time_;
 
   Type type_;