Load the save previews in the background

src/openrct2/Context.cpp

@@ -160,6 +160,8 @@ namespace OpenRCT2
         std::thread::id _mainThreadId{};
         Timer _forcedUpdateTimer;
 
+        BackgroundWorker _backgroundWorker;
+
     public:
         // Singleton of Context.
         // Remove this when GetContext() is no longer called so that
@@ -1333,6 +1335,8 @@ namespace OpenRCT2
                 _ticksAccumulator -= kGameUpdateTimeMS;
             }
 
+            _backgroundWorker.dispatchCompleted();
+
             ContextHandleInput();
             WindowUpdateAll();
 
@@ -1366,6 +1370,8 @@ namespace OpenRCT2
                     tweener.PostTick();
             }
 
+            _backgroundWorker.dispatchCompleted();
+
             ContextHandleInput();
             WindowUpdateAll();
 
@@ -1550,6 +1556,11 @@ namespace OpenRCT2
         {
             return _timeScale;
         }
+
+        BackgroundWorker& GetBackgroundWorker() override
+        {
+            return _backgroundWorker;
+        }
     };
 
     Context* Context::Instance = nullptr;