Inconstant Cluster/Decluster zoom levels

Reported on the forum here: https://groups.google.com/forum/#!topic/cesium-dev/HTAJnvct838

> When zooming in and out with billboard clusters, the levels at which billboards cluster and uncluster is inconsistent. It seems that pixelRange is applied to determine when to group billboards into a cluster, but the cluster is "sticky" when zooming in and does not break apart until some zoom level where a significantly larger pixel range is seen.

> This can be reproduced with the clustering demo. Here are steps:

1. Set "Pixel Range" to 100 and "Minimum Cluster Size" to 2.
2. Zoom in to get to a place where 2 billboards are nearby and may be clustered
3. Slowly zoom out from the 2 billboards and notice the last zoom level before the billboards combine.
4. Slowly zoom in and notice that you need to keep zooming past the level from above to get the billboards to uncluster.

If this is intended behavior, we can go ahead an close this issue.

Start with the clustering demo and reproduce the reported sequence using Pixel Range 100 and Minimum Cluster Size 2. Compare the zoom thresholds when billboards cluster while zooming out and uncluster while zooming in; done means both transitions occur consistently at the same effective range, or the intended behavior is documented.

Written by the indexing model from the issue text.