feat(Gsplat): IBL shadows support for Gaussian Splats

[raymondyfei]

Summary

This PR extends the IBL shadow voxelizer to support GaussianSplattingMesh on WebGL and WebGPU.

Splats are rasterized as quads; edge-on quads produce no fragments, so they can’t be voxelized like triangle meshes in one pass. Splats are drawn three times (one per world axis) so each splat is face-on from at least one direction.

computeVoxelSplatWorldPos (shared GLSL/WGSL) orients each quad from the current view using splat rotation/scale data.

Opt-in memory: voxelization needs extra rotation/scale textures. Set needsRotationScaleTextures = true when loading splats if you want to enable this GSplat IBL shadow feature (playground #DAID4Q). We'll log an error and early exit if neither keepInRam = true nor needsRotationScaleTextures = true is set when enabling the IBL shadow and adding the GSplat as a caster.

Changes (high level)

• Shaders: gaussianSplattingVoxel vertex/fragment (GLSL + WGSL); WGSL targets texture_storage_3d.
  gaussianSplatting include adds computeVoxelSplatWorldPos and struct fields behind IS_FOR_VOXELIZATION.
• API: GaussianSplattingMaterial.makeVoxelRenderingMaterial(); mesh/base needsRotationScaleTextures; SPLATLoadingOptions.needsRotationScaleTextures for load-time allocation.
• Pipeline: iblShadowsVoxelRenderer custom render path + shared tri-axis view matrices; iblShadowsRenderPipeline toggles the flag when splat casters are added/removed.
• Tests: playground visualization + WebGL/WebGPU reference images.

[sebavan]

/azp run

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[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[Popov72]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[bjsplat]

Please make sure to label your PR with "bug", "new feature" or "breaking change" label(s).
To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

Snapshot stored with reference name:
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[bjsplat]

Interaction tests

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/18331/merge/testResults/interactionplaywright/index.html

[bjsplat]

WebGL2 visualization test reporter:

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/18331/merge/testResults/webgl2playwright/index.html

[bjsplat]

Visualization tests for WebGPU

https://snapshots-cvgtc2eugrd3cgfd.z01.azurefd.net/refs/pull/18331/merge/testResults/webgpuplaywright/index.html