Stop calling getNextDrawable on buffer swap to avoid CPU stalls

[frenzibyte]

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getNextDrawable is a blocking call, which doesn't return until a metal drawable is available or timed out after a full second has passed (and returns null).

Calling the function after a buffer swap is a bad practice as the game may execute other logic that isn't related to rendering (more specifically, doesn't require a metal drawable).

Therefore I've moved the call to only happen on the first draw call of the frame (indicated by the buffer swap). This improves Metal performance quite a lot for our game.

Before:

After: