feat: Random airfield spawning

src/sim/airfield.hpp

@@ -12,11 +12,12 @@ class Airfield : public WorldObject
 {
 private:
 	vec2 _position;
+	int _ttl;
 
 public:
-	Airfield(vec2 position) : _position(position) {}
+	Airfield(vec2 position, int ttl) : _position(position), _ttl(ttl) {}
 
-	void tick(float timestep, vec2 bounds) override;
+	void tick(float timestep, World *world) override;
 	void draw(Renderer *rend) override;
 };