Enable billboard depth testing improvements for non clamped to ground billboards

See https://github.com/AnalyticalGraphicsInc/cesium/pull/6621#issuecomment-401419591

There are other use cases where this feature can be helpful than just when billboards are clamped to ground. We can consider either enabling it with a flag, or enabling it all of the time (but we'd have to make sure there's not an impact on performance).