feat: Approaches and referenced airfields

src/sim/airfield.hpp

@@ -20,9 +20,12 @@ private:
 	int _next_spawn;
 	float _rw_heading;
 
+	int _pending_arrivals;
+
 	int const _MIN_TAKEOFF_DELAY = 600;
 	int const _MAX_TAKEOFF_DELAY = 3000;
 	int const _RW_LENGTH = 6;
+	float const _APPROACH_DIST = 20.0f;
 
 public:
 	Airfield(AFSpawner *parent, vec2 position, int ttl, float rw_heading)
@@ -32,6 +35,10 @@ public:
 	}
 
 	vec2 get_position() { return _position; }
+	vec2 get_approach_point();
+	void going_to() { ++_pending_arrivals; }
+	void arrived_at() { --_pending_arrivals; }
+	bool is_active() { return _ttl > 0; }
 
 	void tick(float timestep, World *world) override;
 	void draw(Renderer *rend) override;