add a method to `ScreenStack` which will peek and render the current `Screen` if it is not null

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at the moment i feel like too much ScreenStack logic is relied upon the end user, so maybe we just combine all of its logic into a process() or update() method that gets called every frame, instead of manually peeking it and rendering it, but i'll re-evaluate this choice at a later time.

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this will make the ScreenStack more resilient and self-sufficient so that end users don't have to worry about rendering a null Screen manually, but still allow them to peek the current Screen