Manage multi scene

Manage multi scene

 - Create position and check if user is in position to switch in the next scene.
 - When scene is load, we load all scene in the layer of name "exit".
 - Layer "exit" of map.json have a parametter "exitSceneKey" to identify next scene.
 - Add layer "start", the player could start in the scene on the object present in the layer of name "start".