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);
```
The only reason I put this is incase I setup the camera to follow a sprite, then remove that sprite (for any reason). There could be cycles where there is no parent, didn't want it to throw an error.