Hiding game window until run loop begins

Fixing the platform and window to not show the window until the run loop starts. 

First, this is more accurate to XNA's actual behavior. If you override BeginRun in XNA and set a breakpoint, you'll note the window has still not been shown. That is now replicated with this fix.

Second, this just looks better. The window appears with the correct size based on the GraphicsDeviceManager (instead of the defaults) and doesn't appear empty since it isn't shown until after OpenGL has been initialized.