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/IGraphicsDevice.cs

@@ -34,7 +34,7 @@ namespace OpenRA
 
 	public enum BlendMode { None, Alpha, Additive, Subtractive, Multiply }
 
-	public interface IGraphicsDevice
+	public interface IGraphicsDevice : IDisposable
 	{
 		IVertexBuffer<Vertex> CreateVertexBuffer(int length);
 		ITexture CreateTexture(Bitmap bitmap);
@@ -58,8 +58,6 @@ namespace OpenRA
 		void DisableDepthBuffer();
 
 		void SetBlendMode(BlendMode mode);
-
-		void Quit();
 	}
 
 	public interface IVertexBuffer<T>