Ellipsoid precision

Find the intersection of the ray from the eye to a sphere with the ellipsoid's maximum radius. If it intersects, march the ray forward and find the intersection with the ellipsoid. This should fix some ellipsoids like in #899. 

The artifacts will still occur for ellipsoids where the ratio of the maximum radius to the minimum radius is greater than 100. Marching rays the rays closer to the ellipsoid doesn't help.

You can test this in the Volume Sandcastle example by dividing an ellipsoid's smallest radius by 100.0. In master, you'll see the artifacts. If you divide by a higher value, say 1000.0, there are still artifacts.

http://cesium.agi.com/Cesium/Apps/Sandcastle/index.html?src=Volumes.html

@emackey Can you verify this fixes #899?