Update Cesium

This pull request brings us in sync with the latest changes in Cesium's `master` and `kml` branches.  It also includes some changes to National Map to take advantage of new Cesium features:

* Removed the limit on the number of data sources that can be shown simultaneously, because Cesium now supports an unlimited number of imagery layers regardless of hardware.
* Removed the code checking for intersection between the camera and terrain.  Cesium now keeps the camera above the terrain surface automatically (and quite nicely).