game world boundary, pixel-to-meter mapping

[thoraxe]

Box2D worlds have no boundaries, but we need a boundary for the area of play. For now, the simplest thing to do is:

1. decide on a size of the play area in meters/kms/whatevers
2. figure out a mapping of the size of the play area to the pixels of the client "screen"
3. create a physical box2d boundary at the edges of the play area
4. make players bounce off the boundary

See https://stackoverflow.com/questions/8020093/box2d-world-size for creating boundaries on the play area