Race condition in viewer.zoomTo

[mramato]

1. Run one of the [Geometry examples](http://localhost:8080/Apps/Sandcastle/index.html?src=Circles and Ellipses.html&label=All)
2. Add viewer.scene.morphTo2D(); to the bottom.

When you run the app, you'll get a crash because viewer.zoomTo ends up calling viewBoundingSphere during the morph. We need to detect that the morph is in process and wait for it to complete before zooming.

Here's the callstack.

DeveloperError: viewBoundingSphere is not supported while morphing.
Error
   at DeveloperError (http://localhost:8080/Source/Core/DeveloperError.js:43:13)
   at Camera.prototype.viewBoundingSphere (http://localhost:8080/Source/Scene/Camera.js:2372:13)
   at updateZoomTarget (http://localhost:8080/Source/Widgets/Viewer/Viewer.js:1584:13)
   at Viewer.prototype._postRender (http://localhost:8080/Source/Widgets/Viewer/Viewer.js:1546:9)
   at Event.prototype.raiseEvent (http://localhost:8080/Source/Core/Event.js:146:17)
   at render (http://localhost:8080/Source/Scene/Scene.js:1432:9)
   at Scene.prototype.render (http://localhost:8080/Source/Scene/Scene.js:1440:13)
   at CesiumWidget.prototype.render (http://localhost:8080/Source/Widgets/CesiumWidget/CesiumWidget.js:650:13)
   at render (http://localhost:8080/Source/Widgets/CesiumWidget/CesiumWidget.js:68:25)