Tweaks from National Map

One serious bug:
* Fixed a bug that caused a newly-added imagery layer to be added _twice_ to each terrain tile.  Only terrain tiles that already existed prior to adding the imagery layer were affected.

And two changes to improve compatibility on old browsers.  We use a significant and increasing portion of Cesium code with Leaflet on IE9 and 10, and I think these changes are generally useful (e.g. for NORAD Tracks Santa):
* Added `FeatureDetection.supportsWebWorkers`.
* Updated `loadWithXhr` to support busted old browsers.