Added Some Art Assets From Chris

- Added Backdrop of the forest level. It's currently attached to the
  camera, We'll need a script that make it move a little bit. It still
  look kinda strange without the rest of the map and I dont think it's a
  final version either.

- Added the forest tiles in the Environment folder. They're not in the
  scene yet, Anybody is welcome to start adding them (though I'd wait for
  the final layout !).