Create reference implementation for BENTLEY_materials_point_style

[markschlosseratbentley]

Description

This PR adds support for the BENTLEY_materials_point_style glTF vendor extension. The specification PR (now merged) can be found here and the specification itself currently lives here.

This PR allows CesiumJS to process and apply the above extension when loading glTF files. This means point primitives will be able to have a diameter property specified and respected in CesiumJS when loaded via glTF.

To test these changes locally:

• Clone this branch.
• Build CesiumJS and Sandcastle.
• Run the dev server locally.
• Connect to the dev server using your local browser, clicking on Sandcastle.
• Navigate to the Styled glTF Points example.

Here is a screenshot of test file points-r5-g8-b14-y10.gltf being rendered in CesiumJS

This change further enables CAD-style workflows in CesiumJS.

Issue number and link

Fixes #12891

Testing plan

This has been tested by:

• Adding a Cesium Sandcastle sample.
• Adding a unit test.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
• I have added or updated unit tests to ensure consistent code coverage
• I have updated the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @markschlosseratbentley!

✅ We can confirm we have a CLA on file for you.

[Copilot]

Pull request overview

This PR implements support for the proposed BENTLEY_materials_point_style glTF extension, enabling point primitives in glTF files to specify and respect a diameter property in CesiumJS.

Key changes:

• Added glTF loader support to parse and validate the BENTLEY_materials_point_style extension
• Implemented shader modifications to render points with specified diameters as circles
• Created a Sandcastle example demonstrating the new functionality

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/sandcastle/gallery/styled-gltf-points/sandcastle.yaml	Metadata for new Sandcastle example
packages/sandcastle/gallery/styled-gltf-points/main.js	Sandcastle example loading styled point glTF model
packages/sandcastle/gallery/styled-gltf-points/index.html	HTML wrapper for the Sandcastle example
packages/engine/Specs/Scene/GltfLoaderSpec.js	Unit tests for extension loading and validation
packages/engine/Source/Shaders/Model/ModelVS.glsl	Vertex shader support for point diameter
packages/engine/Source/Shaders/Model/ModelFS.glsl	Fragment shader to render points as circles
packages/engine/Source/Scene/ModelComponents.js	Added pointDiameter property to Material component
packages/engine/Source/Scene/Model/MaterialPipelineStage.js	Pipeline stage to configure point diameter uniform
packages/engine/Source/Scene/GltfLoader.js	Extension parsing and validation logic
Specs/Data/Models/glTF-2.0/StyledPoints/points-r5-g8-b14-y10.gltf	Test glTF file with styled points
CHANGES.md	Changelog entry for the new feature
Apps/SampleData/models/StyledPoints/points-r5-g8-b14-y10.gltf	Sample glTF file for Sandcastle example

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[lilleyse]

Looks good. I left some comments in CesiumGS/glTF#91, but those may or may not affect this PR.

[ggetz]

@jjhembd Could you take a review pass on the code here, and merge once happy?