Fixed IDisposable implementation and usage.

- Implement IDisposable interface correctly, with sealed classes where possible for simplicity.
- Add using statement around undisposed local variables.

OpenRA.Game/Support/PerfHistory.cs

@@ -105,7 +105,7 @@ namespace OpenRA.Support
 		}
 	}
 
-	public class PerfSample : IDisposable
+	public sealed class PerfSample : IDisposable
 	{
 		readonly Stopwatch sw = Stopwatch.StartNew();
 		readonly string Item;

OpenRA.Game/Support/PerfTimer.cs

@@ -16,7 +16,7 @@ using System.Threading;
 
 namespace OpenRA.Support
 {
-	public class PerfTimer : IDisposable
+	public sealed class PerfTimer : IDisposable
 	{
 		readonly string name;
 		readonly float thresholdMs;

OpenRA.Game/Support/Program.cs

@@ -57,7 +57,7 @@ namespace OpenRA
 
 			if (Game.Settings.Debug.ShowFatalErrorDialog && !Game.Settings.Server.Dedicated)
 			{
-				Game.Renderer.Device.Quit();
+				Game.Renderer.Device.Dispose();
 				Platform.ShowFatalErrorDialog();
 			}
 		}