Make ModelExperimental handle classification

[j9liu]

This PR adds support for classification models in ModelExperimental. This was accomplished without needing to create an equivalent of ClassificationModel. Instead, a ClassificationModelDrawCommand is created to handle the special classification commands.

Sandcastle for testing (updated for ModelExperimental -> Model). The tileset is clipped with a clipping plane so you can see the terrain underneath.

I also found #10160 while looking through issues related to classification, and I confirmed that the sandcastle (updated for ModelExperimental -> Model) doesn't crash with the changes in this PR.

Summary:

• Added ClassificationModelDrawCommand. This is modeled after the commands in Vector3DTilePrimitive.
• Added classificationType to ModelExperimental.
• Model3DTileContent now passes the classification option to ModelExperimental.
• Updated specs for various pipeline stages to accommodate the attribute / shader differences when rendering classification models.
• Updated ModelExperimental specs for setting / changing classificationType.
• Updated Model3DTileContent with the render specs from Batched3DModel3DTileContentClassificationSpec.

[cesium-concierge]

Thanks for the pull request @j9liu!

• ✔️ Signed CLA found.

Reviewers, don't forget to make sure that:

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

[ptrgags]

@j9liu a couple initial comments for now, most notably that many tests are failing at least on Linux.

[j9liu]

@ptrgags updated with the ModelExperimental -> Model changes!

[ptrgags]

@j9liu The classification model is working nicely! And I can confirm that this Sandcastle is now working again.

I had a few comments/questions before we merge

[ptrgags]

Just for good measure, I tried swapping out the model in your Sandcastle example with other models. When I try it with the Microcosm example, I get a shader compilation error:

Sandcastle

RuntimeError: Fragment shader failed to compile.  Compile log: ERROR: 0:51: 'v_texCoord_0' : undeclared identifier
ERROR: 0:51: 'texture2D' : no matching overloaded function found
ERROR: 0:51: 'r' :  field selection requires structure or vector on left hand side
ERROR: 0:59: 'texCoord_0' :  no such field in structure
ERROR: 0:59: 'texture2D' : no matching overloaded function found
ERROR: 0:59: 'r' :  field selection requires structure or vector on left hand side

is this expected? I also tried BoxTextured and that worked fine

[j9liu]

@ptrgags I pushed a fix -- I didn't realize that FeatureIdPipelineStage used texcoords, and those weren't being processed in GeometryPipelineStage for classification models.

[ptrgags]

I can confirm that the fix does fix things for Microcosm!

Updated Sandcastle

[j9liu]

@ptrgags updated!

[ptrgags]

@j9liu just a couple quick things I noticed about the @experimental tags

(I also see you pushed some commits while I was reviewing, sorry if anything overlaps 😅)

[j9liu]

@ptrgags added the @experimental tags back in!

[ptrgags]

Looks good, thanks @j9liu!