Overlapping image material z-fighting bug

Reported on the forum: https://groups.google.com/forum/#!topic/cesium-dev/EKRqbL2TJJU


This problem doesn't happen in 2D or for polygons with a PerInstanceColorMaterial

``` javascript
var czml = [{
"id" : "document",
"name" : "CZML Geometries: Polygon",
"version" : "1.0"
}, {
"polygon" : {
"positions" : {
"cartographicDegrees" : [
-108.0, 25.0, 0,
-100.0, 27.0, 0,
-100.0, 32.0, 0,
-108.0, 30.0, 0
]
},
"height": 0,
"material" : {
"image": {
"image": { "uri": "../images/Cesium_Logo_Color.jpg" }
}
}
}
},{
"polygon" : {
"positions" : {
"cartographicDegrees" : [
-109.0, 26.0, 0,
-101.0, 28.0, 0,
-101.0, 33.0, 0,
-109.0, 31.0, 0
]
},
"height": 0,
"material" : {
"image": {
"image": { "uri": "../images/Cesium_Logo_Color.jpg" }
}
}
}
}];

var viewer = new Cesium.Viewer('cesiumContainer');
var dataSource = Cesium.CzmlDataSource.load(czml);
viewer.dataSources.add(dataSource);
viewer.zoomTo(dataSource);
```

Start by running the supplied Cesium.Viewer and CzmlDataSource.load reproduction with the two overlapping textured polygons, then compare it with 2D and PerInstanceColorMaterial cases. Trace the polygon rendering path from the viewer and determine how completion should be verified: overlapping image-material polygons no longer visibly z-fight.

Written by the indexing model from the issue text.