title: Cesium path onto terrain: line connecting 2 points goes under the terrain
body: <p>I have a path moving over time.
  I use Cesium.sampleTerrain to get positions elevation and drape them on the terrain.
  The problem is that, even if all points are on the terrain, the line connecting 2 points sometimes goes under the terrain.
  How can I do to drape also connecting lines on the terrain?</p>
  
  <p>Here is my code:</p>
  
  <pre><code>var promise = Cesium.sampleTerrain(terrainProvider, 14, positions);
  Cesium.when(promise, function(updatedPositions) {
      var cartesianPositions = Cesium.Ellipsoid.WGS84.cartographicArrayToCartesianArray(updatedPositions);
      var sample = new Cesium.SampledPositionProperty();
      sample.setInterpolationOptions({
          interpolationDegree : 3,
          interpolationAlgorithm : Cesium.HermitePolynomialApproximation
      });
  
      $(cartesianPositions).each(function(index, cartPosition) {
          var time = Cesium.JulianDate.addSeconds(start, index*10, new Cesium.JulianDate());
          sample.addSample(time, cartPosition);
  
      })
  
      var target = viewer.entities.add({
          position: sample,
            path: {
              resolution: 60,
              material:Cesium.Color.BLUE,
              width: 4,
              trailTime: 422*10,
              leadTime: 0
            }
      });
  
  });
  </code></pre>