improve large coordinate precision issue

[WilliamLiu-1997]

Relate to #287

Summary

Improve far-from-origin precision for transformed SplatMesh scenes by rebasing internal splat generation, accumulation, and sorting to view-relative coordinates.

Internally, splat centers are no longer propagated as long-lived absolute world-space values by default. The pipeline now prefers centers relative to the current viewOrigin / accumulator origin and only converts back to world space where external semantics require it.

What Changed

• Rebased accumulator output to viewOrigin for both packed and extended splat paths
• Updated depth sorting to use rebased centers
• Applied render-time origin restoration consistently in SparkRenderer
• Added support for generators that already emit view-relative centers
• Rebased SplatMesh generation before large world translations lose precision

Here are examples：

Example that fixed precision issue
https://threejs-3d-tiles-renderer-mynn6oxso-williamliu1997s-projects.vercel.app/

Example that current Spark has (you can see model is flickering)
https://threejs-3d-tiles-renderer-7rj9n6oeu-williamliu1997s-projects.vercel.app/