Polyline with arrow material is blurry

Reported on the forum: https://groups.google.com/forum/?hl=en#!topic/cesium-dev/sFOMs4XMJQk

It's particularly noticeable when the polyline has a large width:

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

var purpleArrow = viewer.entities.add({
name : 'Purple straight arrow at height',
polyline : {
positions : Cesium.Cartesian3.fromDegreesArrayHeights([-75, 43, 500000,
-125, 43, 500000]),
width : 100,
arcType : Cesium.ArcType.NONE,
material : new Cesium.PolylineArrowMaterialProperty(Cesium.Color.PURPLE)
}
});
```

Use the supplied JavaScript Viewer example as the entry point and reproduce the blur with width 100 and PolylineArrowMaterialProperty. Trace the polyline-arrow rendering path; done when the arrow remains sharp at large widths and the example still renders correctly.

Written by the indexing model from the issue text.