Refactor Overworld loading to use async tasks

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.

src/app/zelda3/sprite/sprite.h

@@ -279,7 +279,7 @@ static const std::string kSpriteDefaultNames[]{
 class Sprite : public GameEntity {
  public:
   Sprite() = default;
-  Sprite(std::vector<uint8_t> src, uint8_t overworld_map_id, uint8_t id,
+  Sprite(const std::vector<uint8_t>& src, uint8_t overworld_map_id, uint8_t id,
          uint8_t x, uint8_t y, int map_x, int map_y)
       : map_id_(static_cast<int>(overworld_map_id)),
         id_(id),