Add support for GLTF extension: EXT_mesh_primitive_edge_visibility

[danielzhong]

Description

This PR implements the proposed GLTF extension EXT_mesh_primitive_edge_visibility, enabling edge rendering for glTF mesh primitives. It adds multi-pass rendering to avoid z-fighting, and implements decoding, creation, and rendering of both hard and silhouette edges:

• Parsing and handling mesh primitive data of visibility (edge type: HIDDEN, SILHOUETTE, HARD, REPEATED) and silhouette normal.
• Hard edge & silhouette edge rendering.
• Multi-pass rendering path (CESIUM_3D_TILE_EDGES) to draw edges before surfaces (CESIUM_3D_TILE).
• Feature ID/Depth integration to ensure edges display correctly without Z fighting and overlapping.

More details: Edge Visibility

Example
(green edge = hard edge; red edge = silhouette edge):

Issue number and link

#12765

Testing plan

• Added Unit test for Edge Frame Buffer, MRT & Shader Color Attachments, Decoding logic, Shader Generation, etc.

• Real Time Demo

• Local Test File:
  super_simple_3.bim-tiles.zip

How to Run:

1. Download the file and host: http-server ./ --cors=X-Correlation-Id
2. Open sandcastle and paste:

const viewer = new Cesium.Viewer("cesiumContainer", {
 
  infoBox: false,
 
  selectionIndicator: false,
 
  shadows: true,
 
  shouldAnimate: true,
 
});
const tileset = await Cesium.Cesium3DTileset.fromUrl(
 
  "http://172.20.20.20:8081/tileset.json"
 
);
 
viewer.scene.primitives.add(tileset);
 
await viewer.zoomTo(tileset);

Make sure change your local host address in the code!

((green edge = hard edge; red edge = silhouette edge):)

🔍Review Notes

• The glTF extension’s material support is not included in this PR. I'm planning to set all edges default to black.
• Updated getWebGLStub.js to support MRT in coverage tests.
• This PR is ready to review!

Bug Fix Summary

• Edge rendering was incorrect because GL_LINES and edge primitives shared the same VAO. Fixed by creating a separate VAO for edge rendering.
• Added an edge-rendering pipeline stage in modelFS.glsl. The model runtime primitive couldn’t detect the render tag (it runs too early), so I introduced a uniform flag to detect when the pipeline stage is active.
• MRT couldn’t clear buffers/color attachments. Fixed by using clearCommand in EdgeFramebuffer.js.
• MRT was failing unit tests. Fixed by updating getWebGLStub.js to support MRT in coverage tests.
• Silhouette edges bug: in top-down views, the silhouette normal calculation wasn’t accurate enough.

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, @danielzhong!

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