Fix polygon jitter

Fixes #358.

On the website, open the [Polygon Sandcastle example](http://cesium.agi.com/Cesium/Apps/Sandcastle/index.html?src=Polygons.html).  Zoom in very close (think street level) to the corner of one of the polygons (I use the cyan one).  It will jitter, especially when you tilt the camera for a horizon view.

Open the same example [locally](http://localhost:8080/Apps/Sandcastle/index.html?src=Polygons.html), and the polygon will not jitter when you zoom in.