Multiwindow

Adding support for multiple GameWindow(s) within a single game.

This is only implemented for WINDOWS && DIRECTX

Non-primary windows do not handle any input and for input events to reach the users game the primary window must be active. This can be improved at a later time when it is a needed feature.

The existing Game / Platform / GraphicsDevice code is largely unchanged and only deals directly with the standard, primary window which it also creates.

Additional windows can be created by the users game code like:
            GameWindow.Create(Game, Width, Height);

To allow rendering to the window, the user creates a swap chain:
            SwapChain = new SwapChainRenderTarget(windowHandle: Window.Handle,
                                                graphicsDevice: _device,
                                                width: Window.ClientBounds.Width,
                                                height: Window.ClientBounds.Height,
                                                mipMap: false,
                                                presentInterval: PresentInterval.Default,
                                                surfaceFormat: SurfaceFormat.Color,
                                                depthFormat: DepthFormat.Depth16,
                                                preferredMultiSampleCount: 1,
                                                usage: RenderTargetUsage.DiscardContents);

Secondary windows must be rendered before the primary, and is done by embedding code like this within your draw method:

    MyGame::Draw(GameTime)
    {
            _device.SetRenderTarget(SwapChain);

            // Draw stuff...

            SwapChain.Present();

            // Setup to draw to primary window like this...
            _device.SetRenderTarget(null);

            // Draw to primary window
            // Done.
            base.Draw(gameTime);
    }