ModelGraphics silhouette not visible in SceneMode.SCENE2D

Setting the silhouetteSize of a ModelGraphics should display a silhouette, but it does not if the Scene is set to SceneMode.SCENE2D.

Reference the example below and switch between 3D and 2D to see the issue.

Sandcastle example:
[Silhouette example](https://sandcastle.cesium.com/index.html#c=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)

Start with the linked Sandcastle silhouette example and compare ModelGraphics silhouetteSize behavior while switching between 3D and SceneMode.SCENE2D. Trace the ModelGraphics silhouette handling and scene-mode path; done means the silhouette is visible in both modes and the example behaves consistently.

Written by the indexing model from the issue text.