Don't zoom past the surface while double-click zooming.

Fixes #275.

The problem was that we were picking the ellipsoid, not the terrain surface.  In the common case that the ellipsoid is below the terrain surface, the camera might be under the ellipsoid as well, in which case we'd pick the opposite side of the Earth and zoom 2/3 of the way to that.  Fortunately, picking the terrain surface recently became trivial in Cesium.