Remove STORAGE_ORDER shader define

[slimbuck]

Followup to #8487.

The STORAGE_ORDER shader define was used to branch between a storage buffer and a texture-based fallback for splat order data in the WGSL shader. Since WGSL is only used under WebGPU, and WebGPU always uses a storage buffer for order data, the texture fallback was dead code.

Changes

• Remove #ifdef STORAGE_ORDER branching from the WGSL gsplatSource shader chunk, keeping only the var<storage, read> path
• Remove setDefine('STORAGE_ORDER', ...) calls from GSplatInstance.setMaterialOrderData() and GSplatRenderer.configureMaterial()

No functional change — the GLSL (WebGL) path already uses texture-only order and was unaffected by this define.

[Copilot]

Pull request overview

Removes the now-dead STORAGE_ORDER WGSL shader define and associated branching for GSplat order data, aligning the WGSL/WebGPU path with the fact that WebGPU always supplies order data via a storage buffer.

Changes:

• Simplify WGSL gsplatSource to always read splatOrder from var<storage, read> array<u32>.
• Remove setDefine('STORAGE_ORDER', ...) from GSplat material configuration paths.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/scene/shader-lib/wgsl/chunks/gsplat/vert/gsplatSource.js	Removes texture fallback and always reads order from storage buffer in WGSL.
src/scene/gsplat/gsplat-instance.js	Stops setting STORAGE_ORDER define when binding order buffer/texture.
src/scene/gsplat-unified/gsplat-renderer.js	Stops setting STORAGE_ORDER define during unified renderer material configuration.

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