globe.pick() sometimes returns a hit which isn't closest

This occurs when geometry is overlapping. The points circled in red are actually obscured by the mountain in front so shouldn't have been picked.


I ran this repro from the current Cesium sandcastle with a relatively small window size (as pictured) on Ubuntu Chrome.

```
var viewer = new Cesium.Viewer('cesiumContainer');
var camera = viewer.camera;
var scene = viewer.scene;
var globe = scene.globe;

var cesiumTerrainProviderMeshes = new Cesium.CesiumTerrainProvider({
url: '//assets.agi.com/stk-terrain/world',
requestWaterMask: true,
requestVertexNormals: true
});
viewer.terrainProvider = cesiumTerrainProviderMeshes;

var target = new Cesium.Cartesian3(-2489625.0836225147, -4393941.44443024, 3882535.9454173897);
var offset = new Cesium.Cartesian3(-6857.40902037546, 412.3284835694358, 2147.5545426812023);
viewer.camera.lookAt(target, offset);
viewer.camera.lookAtTransform(Cesium.Matrix4.IDENTITY);

function runTest() {
console.log("Running test now!");
var n = 200;
for (var y = 0; y <= n; ++y) {
for (var x = 0; x <= n; ++x) {
var name = "<" + x + "," + y + ">";
var windowCoordinates = {
x: x * window.innerWidth / n + Math.random(),
y: y * window.innerHeight / n + Math.random()
};
var ray = camera.getPickRay(windowCoordinates);
var intersection = globe.pick(ray, scene);
if(!Cesium.defined(intersection)){
continue;
}

viewer.entities.add({
name: name,
position: intersection,
billboard: {
verticalOrigin: Cesium.VerticalOrigin.BOTTOM,
scale: 0.3,
image: '../images/facility.gif'
}
});

}
}
}

console.log("Just waiting for terrain to load...");
setTimeout(runTest, 10000);
```

Possibly related to https://github.com/AnalyticalGraphicsInc/cesium/issues/3411

Start by running the supplied Sandcastle reproduction with terrain enabled and inspect globe.pick(), camera.getPickRay(), and the related issue #3411. Compare picked terrain intersections for overlapping geometry and verify that obscured points are excluded and the closest visible intersection is returned.

Written by the indexing model from the issue text.