[Linux] Window size fix

Work-around for #628

OpenTK on Linux currently does not allow the window to be resized if the border is fixed. We get the resize event for the intended size, then immediately get a resize event for the original size. This was preventing GraphicsDeviceManager.PreferredBackBufferWidth and PreferredBackBufferHeight from having any effect.
Previously reported to OpenTK here http://www.opentk.com/node/3132

The window size currently flip-flops between the default (640x480) and the intended size a few times on Linux due to OpenTK, but finally settles on the intended size.

Also fixed the GraphicsDevice being created far too early on Linux and Windows.