D3D: Return to windowed mode before destroying the swapchain.
D3D: Initialize the renderer in exclusive fullscreen mode if it is enabled.
Remove fullscreen flag from video config, detect it from RendererIsFullscreen() instead.
Renderer: Restore windowed size after exiting fullscreen.
Renderer: Restore exclusive mode after focus has been regained.
Exit fullscreen when user is prompted to confirm stop.

And fix a small typo while we're are it.