decouple waterline from entity

src/Waterline.h

@@ -1,13 +1,14 @@
 #pragma once
 #include "Entity.h"
 
-class Waterline : public Entity {
+class Waterline {
 private:
+  size_t x, y;
   std::vector<std::string> shape;
   void shiftString(std::string &, int direction);
 
 public:
   Waterline();
-  void draw();
+  void draw() const;
   void update();
 };