Different level tiles loaded in 2D

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/1eIeTqEqgsA


1. http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Hello%20World.html&label=Showcases
2. Use the base layer picker to select Bing maps roads
3. Switch to 2D
4. Gradually zoom in on Europe

Eventually you'll see France/Switzerland tiles from level 5 and Germany/UK will be tiles from level 4. This persists for several scroll wheel clicks for me.

I know this is expected behavior in 3D, but in 2D I would expect all tiles to be from the same level.

Reproduce the issue in the Sandcastle Hello World example using the Bing maps roads layer, switching to 2D and zooming over Europe. Then trace the 2D tile-loading path and compare the displayed tile levels; done means the 2D view keeps the visible tiles at a consistent level while zooming.

Written by the indexing model from the issue text.