Clustering changes when zooming/panning in 2D

Reported on the forum here: https://groups.google.com/forum/#!topic/cesium-dev/Kt1BfW0o5rU

In the clustering demo, if you change to 2D mode, zoom in, then pan around, the clustering changes. As stated in the forum posts, while this may be necessary in 3D, in 2D the reclustering is unneeded and can be distracting.

Start by reproducing the behavior in the clustering demo: switch to 2D, zoom in, and pan around, then review the linked Cesium forum discussion for the intended distinction between 2D and 3D. Done means panning and zooming in 2D no longer causes distracting reclustering, while the existing 3D behavior remains appropriate.

Written by the indexing model from the issue text.