Terrain produces unexpected results in rotatable 2D

[likangning93]

var viewer = new Cesium.Viewer('cesiumContainer', {
    terrainProvider: Cesium.createWorldTerrain(),
    sceneMode : Cesium.SceneMode.SCENE2D,
    mapMode2D : Cesium.MapMode2D.ROTATE
});

On current Sandcastle

This is reproducible at least as far back as 1.39 using STK World terrain.

[hpinkos]

Marking this priority - high because we'll need this fixed for our goal of enabling terrain by default

[likangning93]

It's fractal-y as you get closer, which smells kind of like a near/far plane problem.