[Metal] Fix per-point colors in point clouds to match #494

[srmainwaring]

🦟 Bug fix

Follow up to #494

Summary

This fix applies changes proposed by @iche033 to update Metal shaders that were missed from the merged PR.

It is a cherry-pick of 5df748e

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #550 (1b84d59) into main (0310d26) will increase coverage by 0.00%.
The diff coverage is n/a.

❗ Current head 1b84d59 differs from pull request most recent head 69c353e. Consider uploading reports for the commit 69c353e to get more accurate results

@@           Coverage Diff           @@
##             main     #550   +/-   ##
=======================================
  Coverage   53.36%   53.37%           
=======================================
  Files         211      211           
  Lines       20830    20830           
=======================================
+ Hits        11116    11117    +1     
+ Misses       9714     9713    -1     

Impacted Files	Coverage Δ
...e/ignition/rendering/base/BaseGaussianNoisePass.hh	100.00% <0.00%> (+3.33%)	⬆️

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