Enable billboard depth testing improvements for non clamped to ground billboards

[hpinkos]

See #6621 (comment)

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).

[hpinkos]

See this branch: https://github.com/AnalyticalGraphicsInc/cesium/tree/enable-billboard-depth

Currently blocked by #6840 and #6838