cache: mild auto-cache overhaul

Shifted from CacheTask to functionality built into RenderBackend. It was
a lot easier to control behavior this way without having to juggle a ton
of threads and race conditions.

Could likely be multithreaded further.

app/render/backend/renderticketwatcher.h

@@ -31,6 +31,11 @@ class RenderTicketWatcher : public QObject
 public:
   RenderTicketWatcher(QObject* parent = nullptr);
 
+  RenderTicketPtr GetTicket() const
+  {
+    return ticket_;
+  }
+
   void SetTicket(RenderTicketPtr ticket);
 
   bool WasCancelled();