Mario: set scene's background objects

mario/visitors/mario_visitor.hpp

@@ -79,7 +79,7 @@ public:
         // TODO remove coin_block?
         coin_block = coin;
         _scene.addObject(coin);
-        _scene.setZIndex(coin, 1);
+        _scene.moveZJustAboveBackground(coin);
     }
 
     bool hasCoinBlock() {
@@ -96,7 +96,7 @@ public:
 
     void setMushroomBlock(std::shared_ptr<MarioBlock> &mushroom) {
         _scene.addObject(mushroom);
-        _scene.setZIndex(mushroom, 1);
+        _scene.moveZJustAboveBackground(mushroom);
         _moving_objects.push_back(mushroom);
     }