Finalize CesiumJS support for KHR_mesh_primitive_restart

[markschlosseratbentley]

Summary

KHR_mesh_primitive_restart is the successor to the published EXT_mesh_primitive_restart extension. Instead of the EXT's mesh-level primitiveGroups JSON (one glTF primitive per sub-strip plus a separate "combined" accessor), the KHR extension has no JSON payload at all: a primitive's own indices accessor may contain inline "restart" values (the maximum value for the accessor's component type (0xff / 0xffff / 0xffffffff)). Each of these terminates the current strip and begins a new one. This allows many line strips to be batched into a single primitive, avoiding significant glTF JSON bloat for polyline-heavy assets.

We suspect CesiumJS already supports this format de facto, but not officially; there are no tests, no fixture, no docs, and no CHANGES entry.

Current state

• ModelUtility.supportedExtensions already whitelists KHR_mesh_primitive_restart (added in Vector Tiles: Support 3DTILES_content_gltf_vector and EXT_mesh_polygon #13478 for vector tiles / 3DTILES_content_gltf), so extensionsRequired declaring it does not throw.
• We believe the loader never validates the core-glTF prohibition on max-value indices, so inline restart values pass through GltfLoader transparently.
• WebGL 2 should have PRIMITIVE_RESTART_FIXED_INDEX behavior permanently enabled for indexed draws, so restart works on the GPU with zero renderer changes.
• The original EXT primitiveGroups format has explicit handling in getMeshPrimitives.js with fixture + 6 specs in GltfLoaderSpec.js; that support should be retained for backward compatibility (for now, at least).

Proposed work

1. Test assets: add a KHR-format test asset (single LINE_STRIP primitive with inline restart values, extensionsUsed + extensionsRequired). Ideally a styled line-strip asset (BENTLEY_materials_line_style pattern + CUMULATIVE_DISTANCE) to exercise the styling path as well. Keep the existing EXT fixture (Specs/Data/Models/glTF-2.0/MeshPrimitiveRestart/).
2. Specs: add a "loads model with KHR_mesh_primitive_restart" spec to GltfLoaderSpec.js. Keep the 6 existing EXT specs.
3. Docs: update the Model.js JSDoc extension list to add KHR_mesh_primitive_restart linking Add KHR_mesh_primitive_restart KhronosGroup/glTF#2569.
4. CHANGES.md entry? Unsure if necessary.
5. (Optional; is this necessary?) Conformance: buildVectorGltfFromMVT.js emits inline 0xffffffff restart values in LINE_STRIP index buffers but does not seem to declare any restart extension in the generated glTF. Considering adding KHR_mesh_primitive_restart to its extensionsUsed/extensionsRequired.

Known limitations (to document, not necessarily fix)

• WebGL 1: no hardware primitive restart; supporting it would require CPU index-splitting preprocessing. Not implemented.

Related

• Add KHR_mesh_primitive_restart KhronosGroup/glTF#2569 — KHR_mesh_primitive_restart spec (open)
• EXT_mesh_primitive_restart: Performance optimizations and schema updates glTF#100 — revised EXT proposal (superseded by the KHR extension)
• Vector Tiles: Support 3DTILES_content_gltf_vector and EXT_mesh_polygon #13478 — added the supportedExtensions whitelist entry
• Officially document + prove support for KHR_mesh_primitive_restart for line strips iTwin/itwinjs-core#9514 — equivalent effort in iTwin.js (tests + docs; loader already conformant)