Refactored Overworld::LoadSprites to use std::async for concurrent sprite loading, improving efficiency. Modified Overworld::LoadOverworldMaps to wait for all async tasks and check results for errors. Updated Overworld::LoadSpritesFromMap to use mutable_current_graphics() for mutable access to current graphics. Added mutable_current_graphics method to OverworldMap class. Optimized Sprite constructor to take a const reference to avoid unnecessary copying.
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