GroundPrimitives Don't Receive Shadows

Reported [here ](https://groups.google.com/forum/#!topic/cesium-dev/1UlrSfW2ie8)and [here](https://groups.google.com/forum/#!topic/cesium-dev/FLOUH6ndQhc).

See this [Sandcastle](https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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). Notice how the shadow appears on the box and the terrain, but not on the polygon, despite all being enabled for shadows.


Start with the linked Sandcastle reproduction and compare the shadow behavior of the ground primitive, polygon, box, and terrain. Trace the Cesium shadow-rendering path for ground primitives, then verify that enabled ground primitives receive shadows consistently with the other affected geometry in the reproduction.

Written by the indexing model from the issue text.