Add support for Map Engine to work with multiple BGs (from Tiled)

[mbh21]

When creating an RPG-style game, it is often useful to have two BGs with the same scrolling parameters. It would be nice to be able to set two backgrounds in Tiled and then get the map engine to work with both of them.

[alekmaul]

you can do that on your side and change second BG just before your Wait for vblank.
I do that for eyra for example, or Illmoore. The second BG is not managed by the map engine.
The map engine only manages the BG in charge of collision, friction with the world and holes, etc...