Refactor RenderUtil::Update with helper functions

[adlarkin]

Summary

#853 (comment) makes changes that result in RenderUtil::Update being longer than 500 lines of code (yikes!). It looks like several new features were introduced in ign-gazebo4 (thermal camera, lights, etc.) that have added a lot of code to RenderUtil::Update. So, I have introduced some helper functions that can be called in RenderUtil::Update to reduce this method's size.

Once this is merged, we can forward-port this so that all other branches/PRs don't have this issue appear anymore. It's also worth noting that I only introduced helper functions for code that was introduced in ign-gazebo4 so that we (hopefully) don't get any nasty merge conflicts when we merge ign-gazebo3 forward.

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

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[adlarkin]

I've got a few comments/questions for PR reviewers.

[codecov]

Codecov Report

Merging #858 (2964109) into ign-gazebo4 (24d8949) will decrease coverage by 0.00%.
The diff coverage is 42.22%.

❗ Current head 2964109 differs from pull request most recent head d60121c. Consider uploading reports for the commit d60121c to get more accurate results

@@               Coverage Diff               @@
##           ign-gazebo4     #858      +/-   ##
===============================================
- Coverage        65.69%   65.68%   -0.01%     
===============================================
  Files              240      240              
  Lines            17797    17811      +14     
===============================================
+ Hits             11691    11700       +9     
- Misses            6106     6111       +5     

Impacted Files	Coverage Δ
src/rendering/RenderUtil.cc	42.12% <42.22%> (+0.33%)	⬆️

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