Run GC more during loading.

This helps reduce the peak GC size by trimming temporary loading garbage a bit more often, rather than just doing it at the end of loading.

OpenRA.Game/Game.cs

@@ -170,9 +170,13 @@ namespace OpenRA
 
 			worldRenderer = new WorldRenderer(ModData, OrderManager.World);
 
+			GC.Collect();
+
 			using (new PerfTimer("LoadComplete"))
 				OrderManager.World.LoadComplete(worldRenderer);
 
+			GC.Collect();
+
 			if (OrderManager.GameStarted)
 				return;