SDL demo broken since sdl2-2.5

This commit broke the demo:

https://github.com/haskell-game/sdl2/commit/eb40908f24a136dc6f35dbc27c7faed24efbd477#diff-5e4783ffbf4a7445e2672960c2948177

The error is now:

```
demo/Window.hs:22:11: error: Not in scope: ‘windowOpenGL’
   |
22 |         { windowOpenGL = Just $ defaultOpenGL
   |  
```