Improve camera zooming behavior + camera related cleanups

- Now, the camera will now no longer be pulled towards the upper left
  or lower rigtht corner.
- cameraPos was never used as an interpolating vector, cleaned up related code.
- Core::invGlobalScale is now updated whenever Core::globalScale is changed
  (Not a nice solution but it does work)
- Increase cull radius by 10%, should prevent tiles from disappearing when
  zoomed in a lot.