Add silhouettes to ModelExperimental

[j9liu]

This PR adds support for silhouettes in ModelExperimental, like so:

I also found that ModelExperimentalSpec had a lot of invalid unit tests and was missing some for ModelExperimental.color, so I fixed those as well.

Here's the Sandcastle for testing silhouettes.

Testing Notes

• Silhouettes don't work properly for 2D / CV in Model, so they won't work here.
• One bug I still have to figure out: the "Instanced Box" model doesn't work with translucent silhouettes for some reason, even though all the other models work. I can't compare the results to Model either because Model doesn't support GPU instancing, but the silhouette works for this model if the draw command passes are the same (opaque model color + silhouette, translucent model color + silhouette). FIXED

[cesium-concierge]

Thanks for the pull request @j9liu!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[j9liu]

Figured out what the bug was: in CPUStylingStageVS and CPUStylingStageFS, the model wouldn't show up if the pass of the command was translucent, but the feature color wasn't. This was preventing the translucent silhouette from rendering for an opaque model with feature IDs. Should be fixed with the recent commit.

[ptrgags]

@j9liu I had a few comments.

Also I tried the sandcastle and it seems to be rendering only the silhouette:

Though it seems to work fine for invisible models:

[j9liu]

@ptrgags - updated, let me know if the uniform change from float to bool resolved the bug or not.

[j9liu]

@ptrgags ready!

[ptrgags]

@j9liu this is getting close, it works even with custom shaders! though a point cloud without normals failed:

Sandcastle

[j9liu]

@ptrgags updated with a fix for the point cloud crash!

[ptrgags]

Looks good, thanks @j9liu!