Expose stencil buffer to render code.

The intention is to provide a layer for tracking shadow
rendering, so a single bit is sufficient for now.

OpenRA.FileFormats/Graphics/IGraphicsDevice.cs

@@ -53,6 +53,9 @@ namespace OpenRA.FileFormats.Graphics
 
 		void EnableDepthBuffer();
 		void DisableDepthBuffer();
+
+		void EnableStencilBuffer();
+		void DisableStencilBuffer();
 	}
 
 	public interface IVertexBuffer<T>