Do not pre-render each visual's children twice per frame (backport #1309)

[mergify]

🦟 Bug fix

Summary

BaseVisual::PreRender() pre-renders each visual's children twice every frame. It calls T::PreRender(), which resolves to BaseNode::PreRender() and already recurses into the children through the virtual PreRenderChildren(), and then calls PreRenderChildren() again explicitly. Because the duplication happens at every level of the scene graph, it compounds to O(2^depth) redundant traversals: the more deeply nested a scene's model → link → visual hierarchy, the worse it gets.

This patch removes the redundant call so each visual's children are pre-rendered exactly once.

Benchmark results

Measured on the Gazebo GUI process (gz-sim-gui-client, OgreNext + NVIDIA), matched RelWithDebInfo builds, before → after:

World	FPS	Total GUI CPU	BaseVisual::PreRenderChildren (inclusive)
3000 static shapes	33 → 58 (+73%)	~flat	52% → 32%
3000 moving shapes	52 → 64 (+24%)	−30% less CPU	53% → 35%
jetty (~320 models)	~60 (vsync-capped)	−15% less CPU	22% → 10%

The saving shows up as a higher frame rate when the render thread is the bottleneck (the 3000-shape scene was render-bound at 33 FPS) and as lower CPU when the scene already reaches the vsync target (jetty). The relative reduction in PreRenderChildren grows with scene-graph depth, so deeply nested worlds (jetty) see the largest cut.

Backport Policy

• This is safe to backport to the following versions:
  • Jetty
  • Ionic
  • Harmonic
  • Fortress
• This should not be backported
• I am not sure
• Other (fill in yourself)

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed
• All tests passed

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

---

This is an automatic backport of pull request #1309 done by [Mergify](https://mergify.com).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.46%. Comparing base (99031b7) to head (7b284b5).
⚠️ Report is 56 commits behind head on gz-rendering9.

Additional details and impacted files

@@                Coverage Diff                @@
##           gz-rendering9    #1312      +/-   ##
=================================================
+ Coverage          76.03%   76.46%   +0.42%     
=================================================
  Files                180      180              
  Lines              17240    17274      +34     
=================================================
+ Hits               13109    13208      +99     
+ Misses              4131     4066      -65     

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