Silhouette post process does not work on some geometries

Expect silhouette to work on all entities. Does not work for some entities. This is preventing me from using silhouette to visualize selected objects in the view.

Sandcastle example: [example with box, polygon, and polyline](https://sandcastle.cesium.com/#c=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)

Browser: Version 92.0.4515.107 (Official Build) (x86_64)
Operating System: MacOS 11.4

Start with the linked Sandcastle example and reproduce the silhouette post-process failure for the box, polygon, and polyline entities. Compare which entity geometries render the silhouette and use that comparison to locate the relevant rendering path. Done means silhouette visualization works consistently for all entities shown in the example.

Written by the indexing model from the issue text.