Implemented GraphicsDevice.Reset, Windows/DirectX only.

Does the same thing as GraphicsDeviceManager.ApplyChanges within its Windows/DirectX block.
The common situation this is used is for a tool that needs to do xna rendering on a single control and doesn't even have a game/graphicsdevicemanager.
Put the `#if` inside the Reset method...  else you remove the method from the API for other platforms.