Clipping plane not correctly applied to child tiles


```js
var viewer = new Cesium.Viewer('cesiumContainer');

var clippingPlanes = [
new Cesium.ClippingPlane(new Cesium.Cartesian3(0.0, 0.0, -1.0), 0.0)
];

var tileset = viewer.scene.primitives.add(new Cesium.Cesium3DTileset({
url : Cesium.IonResource.fromAssetId(5743),
maximumScreenSpaceError : 2,
clippingPlanes : new Cesium.ClippingPlaneCollection({
planes : clippingPlanes,
edgeWidth : 1.0
})
}));

tileset.readyPromise.then(function() {
var boundingSphere = tileset.boundingSphere;
var radius = boundingSphere.radius;

viewer.zoomTo(tileset, new Cesium.HeadingPitchRange(0.5, -0.2, radius * 4.0));
}).otherwise(function(error) {
console.log(error);
});
```

cc @ggetz

Use the supplied JavaScript reproduction with Cesium.Viewer, Cesium3DTileset, ClippingPlaneCollection, and asset 5743. First confirm how clipping differs between the root and child tiles, then trace the tileset clipping path. Done means clipping is applied consistently to child tiles and the reproduction no longer shows the reported mismatch.

Written by the indexing model from the issue text.