Clustered billboards flicker when transitioning between scene modes

You can see this in the Clustering Sandcastle example: https://cesiumjs.org/Cesium/Apps/Sandcastle/?src=Clustering.html&label=Showcases

Switch the scene mode from 3D to 2D and the billboards flicker.

Reported on the forum here: https://groups.google.com/forum/#!topic/cesium-dev/Wd0P0EnFDjA

Start by reproducing the issue in the Clustering Sandcastle example and switch the scene mode from 3D to 2D while observing the clustered billboards. Use the linked forum report for context, then trace the scene-mode transition and clustering billboard rendering until the transition completes without flicker.

Written by the indexing model from the issue text.