Screen
A screen object provides the basic lifecycle for a game.
Simple games may only need to have one screen. They are a useful way to
isolate different aspects of your game. For example, you could make one
screen display the title and menu, and another screen contain the game
itself.
You can create a screen by using `reify` like this:
```
(def main-screen
(reify p/Screen
(on-show [this])
(on-hide [this])
(on-render [this])))
```