Display list for the maze structure

Objectives are not display-listed as they are removed when walked over.

src/Maze.h

@@ -17,6 +17,7 @@ class Maze {
 public:
     Maze(const std::string& maze_map, unsigned int scaling_factor, const Texture& floor_texture,
          const Texture& wall_texture, const Texture& outside_texture, Model3D& objective);
+    ~Maze();
 
     void draw();
 
@@ -31,6 +32,8 @@ private:
     const Texture& out;
     Model3D& objective;
 
+    GLuint display_list_index = -1;
+
     const double objective_altitude = 0;
     const double wall_height = 2;
     unsigned corner_pos;