Use const reference

Author: Narre

src/object/ispy.cpp

@@ -124,7 +124,7 @@ Ispy::update(float dt_sec)
 }
 
 void
-Ispy::set_sprite_action(std::string action, int loops)
+Ispy::set_sprite_action(const std::string& action, int loops)
 {
   switch (m_dir)
   {

src/object/ispy.hpp

@@ -39,7 +39,7 @@ class Ispy final : public MovingSprite
   virtual void on_flip(float height) override;
 
 private:
-  void set_sprite_action(std::string action, int loops = -1);
+  void set_sprite_action(const std::string& action, int loops = -1);
 
 private:
   enum IspyState {