Fix camera follow when in a scaled parent

When the camera target has a parent in the tree that is scaled, the current implementation does not work. The world scale of the parent needs to be taken into account.

This may not be the best way to fix this, but so far this has worked for me. You can see the current breakage by scaling the world:

```js
game.world.scale.set(3);
```
When would a target not have a parent? In the current implementation it always should, as it has to be a Sprite on the display list. However thinking about it, it would be nice if the Camera could follow a Point object instead of a sprite too - but that can be done another time.