Jitter when clamping to ground

Noticed while reviewing #3903. Here's a sandcastle example which creates a circular flight with altitude of 0 and then clamps to terrain. There is really jerky camera or entity motion as it animates.

``` javascript
var viewer = new Cesium.Viewer('cesiumContainer', {
shouldAnimate: true,
terrainProviderViewModels : [], //Disable terrain changing
infoBox : false //Disable InfoBox widget
});

//Enable lighting based on sun/moon positions
viewer.terrainProvider = Cesium.createWorldTerrain();

//Use STK World Terrain
viewer.terrainProvider = new Cesium.CesiumTerrainProvider({
url : 'https://assets.agi.com/stk-terrain/world',
requestWaterMask : true,
requestVertexNormals : true
});

//Enable depth testing so things behind the terrain disappear.
viewer.scene.globe.depthTestAgainstTerrain = true;

//Set the random number seed for consistent results.
Cesium.Math.setRandomNumberSeed(3);

//Set bounds of our simulation time
var start = Cesium.JulianDate.fromDate(new Date(2015, 2, 25, 16));
var stop = Cesium.JulianDate.addSeconds(start, 360, new Cesium.JulianDate());

//Make sure viewer is at the desired time.
viewer.clock.startTime = start.clone();
viewer.clock.stopTime = stop.clone();
viewer.clock.currentTime = start.clone();
viewer.clock.clockRange = Cesium.ClockRange.LOOP_STOP; //Loop at the end
viewer.clock.multiplier = 1;

//Set timeline to simulation bounds
viewer.timeline.zoomTo(start, stop);

//Generate a random circular pattern with varying heights.
function computeCirclularFlight(lon, lat, radius) {
var property = new Cesium.SampledPositionProperty();
for (var i = 0; i <= 360; i += 45) {
var radians = Cesium.Math.toRadians(i);
var time = Cesium.JulianDate.addSeconds(start, i, new Cesium.JulianDate());
var position = Cesium.Cartesian3.fromDegrees(lon + (radius * 1.5 * Math.cos(radians)), lat + (radius * Math.sin(radians)), 0);
property.addSample(time, position);

//Also create a point for each sample we generate.
viewer.entities.add({
position : position,
point : {
pixelSize : 8,
color : Cesium.Color.TRANSPARENT,
outlineColor : Cesium.Color.YELLOW,
outlineWidth : 3
}
});
}
return property;
}

//Compute the entity position property.
var position = computeCirclularFlight(-112.110693, 36.0994841, 0.03);

//Actually create the entity
var entity = viewer.entities.add({

//Set the entity availability to the same interval as the simulation time.
availability : new Cesium.TimeIntervalCollection([new Cesium.TimeInterval({
start : start,
stop : stop
})]),

//Use our computed positions
position : position,

//Automatically compute orientation based on position movement.
orientation : new Cesium.VelocityOrientationProperty(position),

//Load the Cesium plane model to represent the entity
model : {
uri : '../../SampleData/models/CesiumAir/Cesium_Air.gltf',
minimumPixelSize : 64,
heightReference: Cesium.HeightReference.CLAMP_TO_GROUND
}
});

viewer.trackedEntity = entity;
```

Start by running the supplied Sandcastle example with the circular flight and Cesium_Air.gltf model using HeightReference.CLAMP_TO_GROUND. Trace the terrain-clamping and animation behavior involved, then verify that the camera or entity no longer moves jerkily while the flight loops.

Written by the indexing model from the issue text.