Possible bug when switching between 2D and 3D when terrainProvider has been set

Hi guys,

I just noticed that if a terrainProvider is set and you start in the SCENE2D SceneMode, then switch to SCENE3D the globe has serious artifacts when zoomed out. I am not sure if this is a bug, or I am doing something wrong. I've tried it on multiple computers/operating systems/browsers (Windows 7 (Chrome, IE8), OSX 10.10 (Chrome, Safari)).

Screenshot of issue: http://i.imgur.com/L3gx0E0.png

Steps to reproduce:
1. Load Hello World: https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Hello%20World.html&label=Showcases

2. Paste in the following code and run:
var viewer = new Cesium.Viewer('cesiumContainer',{
    sceneMode: Cesium.SceneMode.SCENE2D});

var cesiumTerrainProviderMeshes = new Cesium.CesiumTerrainProvider({
    url : '//assets.agi.com/stk-terrain/world',
    requestWaterMask : true,
    requestVertexNormals : true
viewer.terrainProvider = cesiumTerrainProviderMeshes;

3. Change sceneMode via the normal UI button to 3D and spin the planet around. You'll see obvious lines/glitches all over the planet as you spin.

Can confirm on Chrome 43.

I opened a new bug https://github.com/AnalyticalGraphicsInc/cesium/issues/2841