Introduce concept of Position on Map

[basicallydan]

At the moment sprites only exist within the world of the viewport. This is fine but it means doing a lot of movement based around the position of the player. It might be better for ease of expansion for sprites to exist independently of the player and have a position on the mountain, and for the viewport to move whenever the player moves.

I.e.

skier.on('move', function () {
  viewport.centerOn(skier.getXPosition(), skier.getYPosition());
});

This means a few new things are much easier to do:

• Cutscenes
• Introducing the sprites of other players
• Arbitrarily adding sprites at certain points in the game rather than just randomly generating them around the player

[basicallydan]

Fully implemented in 8dfe1d5