tutorial: add a few missing “override” qualifiers.

doc/tutorial/04_texture.cpp

@@ -80,7 +80,7 @@ public:
         return true;
     }
 
-    virtual void tick_draw(float seconds, Scene &scene)
+    virtual void tick_draw(float seconds, Scene &scene) override
     {
         WorldEntity::tick_draw(seconds, scene);
 

doc/tutorial/05_easymesh.cpp

@@ -87,7 +87,7 @@ public:
         Ticker::Unref(m_light2);
     }
 
-    virtual void tick_game(float seconds)
+    virtual void tick_game(float seconds) override
     {
         WorldEntity::tick_game(seconds);
 

doc/tutorial/06_sprite.cpp

@@ -46,7 +46,7 @@ public:
         Ticker::Unref(m_camera);
     }
 
-    virtual void tick_game(float seconds)
+    virtual void tick_game(float seconds) override
     {
         for (int i = 0; i < SPRITE_COUNT; ++i)
         {