Move background context to GraphicsDevice.OpenGL

We should not create a background OpenGL context until the application
adds a GraphicsDevice.
Ignore resize events until init is complete

This fixes a crash on startup related to OpenTKGameWindow.Game being
accessed before it is instantiated.
Make the GameWindow visible in BeforeInitialize

OpenTK.NativeWindow is normally invisible, until explicitly made
visible. GamePlatform.BeforeInitialize() is a reasonable place to do
this.