Pixels per meter update

Here are three unrelated commits I'm putting together in this pull request. The real important one is the pixelsPerMeter update.

The use-case I have is a top-down sport game where the arena's meter size is proportional to the number of players, and this requires pixelsPerMeter to behave properly when updated in the middle of a scene.
To be on the safe side, I think this connection should only be made when the body is actually created, for which it should be moved into the `createBody` function.

In practice this may only cause problems when the world is set back to 0, which is a rare case but let's make sure not to crash in that case anyway.