Fix arrow visual indexing

[iche033]

🦟 Bug fix

Summary

Problem: API to change visibility of the ArrowVisual's Head visual was affecting the incorrect child visual's visibility - it updated the Rotation visibility instead. This PR fixes indexing of child nodes in ArrowVisual.

More info:
The index of child nodes have changed after #831 that changed internal data structure from map to vector. With that change, the index number now corresponds to the order in which the child nodes are added.

Updated test to make sure when we request for ArrowVisual's Head, Shaft, and Rotation child visuals, it returns the visual with the correct geometry.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• 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 #889 (97e0b64) into gz-rendering8 (077e1e7) will increase coverage by 0.05%.
The diff coverage is 28.57%.

❗ Current head 97e0b64 differs from pull request most recent head d07d3e3. Consider uploading reports for the commit d07d3e3 to get more accurate results

@@                Coverage Diff                @@
##           gz-rendering8     #889      +/-   ##
=================================================
+ Coverage          75.15%   75.21%   +0.05%     
=================================================
  Files                176      176              
  Lines              16851    16838      -13     
=================================================
  Hits               12664    12664              
+ Misses              4187     4174      -13     

Files Changed	Coverage Δ
include/gz/rendering/base/BaseAxisVisual.hh	62.71% <0.00%> (ø)
include/gz/rendering/base/BaseArrowVisual.hh	76.78% <33.33%> (+14.46%)	⬆️