Fix particle cell ID computation

[deltakosh]

No description provided.

[bjsplat]

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[bjsplat]

Snapshot stored with reference name:
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Test environment:
https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14993/merge/index.html

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https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14993/merge/index.html#WGZLGJ#4600

Links to test babylon tools with this snapshot:

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https://playground.babylonjs.com/?snapshot=refs/pull/14993/merge#BCU1XR#0

[bjsplat]

WebGL2 visualization test reporter:

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14993/merge/testResults/webgl2playwright/index.html

[bjsplat]

Visualization tests for WebGPU (Experimental)
Important - these might fail sporadically. This is an optional test.

https://babylonsnapshots.z22.web.core.windows.net/refs/pull/14993/merge/testResults/webgpuplaywright/index.html