Ground primitive artifact on Mac/Intel

[pjcozzi]

Potentially a precision issue. Maybe a driver bug.

Select "Draw a Polygon" and zoom in to http://cesiumjs.org/Cesium/Apps/Sandcastle/index.html?src=Ground%20Clamping.html&label=Showcases

Only happens in the closest frustum. Note that it is most noticeable with a top-down view, not a horizon view.

This happens before #4751 added dynamically lowering the bottom cap.

This does not happen on Windows/NVIDIA.

[hpinkos]

Also reported here: https://groups.google.com/d/msg/cesium-dev/BHmN6Vj_hgE/eJrENGq3BwAJ

[ggetz]

I can no longer reproduce this on Mac/Intel.