Adjust platform in the tilemap

Move the last platform one tile down, because it was unreachable
by the player.
Fix collision problem with the hotsand

Sometimes, the collision with the hotsand wasn't working. The problem was
the variable countCall wasn't being correctly initialized when the player
died. To solve this problem, this patch initializes this variable inside
the restartGame function.
Change the assets of the menu

Add a new background and buttons to the menu.
Add new assets for the Credits state

Add a new background and a back button in the credits state.