Fixed a couple of problems to zoomToShowLayer

1) If no movement on a cluster, and zoomToShowLayer is called, it does not auto spiderfied and does not call the callback
2) If at the same zoom level, the marker is not panned to the center on the map when it is not visible.
3) It now auto pans a cluster to the centre of the map before spiderfying when the zoom does not change (only panning.)