Fixed the volumetric clouds cloud map not being centered over the world origin (case 1364465).

[anisunity]

This makes sure that the cloud map's center is above (0,0,0)
https://fogbugz.unity3d.com/f/cases/resolve/1364465/

Testing status

Made sure the world center was at the middle of the pattern in the repro case.

I don't think any test will fail (not sure)

[github-actions]

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[JMargevics]

Subtracting 0.5 seem a pretty straightforward fix and if repro case works correctly now, no additional testing is required ✔