Imagery (and terrain) maximum level is limited in certain places In CV mode

In 3D mode, with the following code, I can reach the maximum possible level (25) while zooming.
However in CV, the maximum level I can reach in this area is 15 (as reported by Cesium Inspector's `Show Tile Coordinates`), so the image is blurry.


```
var viewer = new Cesium.Viewer('cesiumContainer', {sceneMode: Cesium.SceneMode.COLUMBUS_VIEW});
var cesiumTerrainProviderMeshes = new Cesium.CesiumTerrainProvider({
url : 'https://assets.agi.com/stk-terrain/world',
requestWaterMask : true,
requestVertexNormals : true
});

viewer.terrainProvider = cesiumTerrainProviderMeshes;

var Cartesian3 = Cesium.Cartesian3;
var cameraPostion = new Cartesian3(4419887.063689054, 560837.210807391, 4555926.042467477);

viewer.camera.setView({
destination: cameraPostion
});

var scene = viewer.scene;
var entity = viewer.entities.add({
position: cameraPostion,
point: {
pixelSize: 10
}
});

viewer.extend(Cesium.viewerCesiumInspectorMixin);
```

Start with the supplied Columbus View reproduction and use Cesium Inspector's “Show Tile Coordinates” to compare imagery and terrain levels with 3D mode at the given camera position. Trace the Columbus View tile selection and maximum-level behavior. Done means the area can reach the available maximum level in CV without the reported blurry imagery.

Written by the indexing model from the issue text.