code: function(center, radius) {
        var vertArrays = this.vertArrays();
        for (var i = 0; i < vertArrays.length; ++i) {
          radius = vertArrays[i].updateSquaredSphereRadius(center, radius);
        }
        return radius;
    }
docs: FIXME: investigate the performance cost of sharing code between updateSquaredSphereRadius and updateProjectionIntervals