inject `Game` instance into every `Screen`

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this will remove the need for the singleton pattern that's present on Game removing the need to call Game.getInstance().setCurrentScreen() and instead we can just access the Game instance that has been injected. Renderables that need access to the Game instance can just have it injected into them through their respective Screen instance also, opposed to accessing the singleton instance.

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not a good idea, a singleton pattern in java is better than dependency injection in this situation it seems