Use texImage2D for gsplat splat-order uploads

[slimbuck]

Summary

On WebGL, the gsplat CPU-sort path uploads a per-frame Uint32Array of sorted
splat indices into an R32UI texture via UploadStream.uploadPBO →
gl.bufferSubData (PBO orphan) → gl.texSubImage2D. For multi-MB integer
textures, this stalls the main thread on Chrome's renderer→GPU IPC.

A ?cpu-sort (WebGPU CPU sort, same JS sorter) vs ?webgl (WebGL2 CPU sort)
comparison on the same splat scene:

	WebGL2	WebGPU CPU-sort
FireAnimationFrame p90	74 ms	2.2 ms
FireAnimationFrame max	93 ms	2.4 ms
Top hotspot	bufferSubData (~25% of CPU)	uploadStaging (negligible)

Switching WebglUploadStream.uploadDirect to call gl.texImage2D directly
(full-buffer, fresh allocation each frame) takes a different — much cheaper —
IPC path on Chrome and eliminates the stall. WebGPU is unchanged.

Changes

• WebglUploadStream.uploadDirect rewritten to call gl.texImage2D directly,
  bypassing Texture.upload() (which uses texSubImage2D after the first
  frame). Wraps the source TypedArray as Uint8Array when the texture's
  pixel type is UNSIGNED_BYTE.
• GSplatWorkBuffer (unified) and GSplatSorter (legacy) construct their
  UploadStream with useSingleBuffer = !device.isWebGPU, so WebGL routes
  through the new uploadDirect. WebGPU continues using the staging path.

Test plan

• WebGL2 gsplat scene (unified + legacy) renders correctly
• WebGPU gsplat scene unchanged (no visual or perf regression)
• Profile confirms main-thread stall is eliminated on WebGL
• Smoke-test on Windows / Linux WebGL2 to confirm no platform regresses

[Copilot]

Pull request overview

Improves WebGL gsplat CPU-sort performance by changing how the per-frame splat-order index buffer is uploaded, avoiding a costly Chrome renderer→GPU IPC path associated with large integer texSubImage2D updates.

Changes:

• Route WebGL gsplat splat-order uploads through UploadStream single-buffer mode to use uploadDirect.
• Rework WebglUploadStream.uploadDirect to perform full uploads via gl.texImage2D (fresh allocation each call), bypassing the engine Texture.upload() path.
• Keep WebGPU behavior unchanged by continuing to use the staging-buffer path.

Reviewed changes

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

File	Description
src/scene/gsplat/gsplat-sorter.js	Switch legacy gsplat sorter uploads to single-buffer mode on WebGL so it uses the new texImage2D direct upload path.
src/scene/gsplat-unified/gsplat-work-buffer.js	Switch unified gsplat work buffer per-frame order uploads to single-buffer mode on WebGL; keep WebGPU on staging.
src/platform/graphics/webgl/webgl-upload-stream.js	Implement direct texture uploads using gl.texImage2D (full upload) to avoid Chrome stalls from texSubImage2D for large integer uploads.

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