Fix repeatedly loading engine when calling sceneFromFirstEngine

[iche033]

🦟 Bug fix

Fixes #959

Summary

Added an better checks to make sure we return the engine if it was already previously loaded instead of loading the engine again.

To test:

gz sim -s -r -v3 --iterations 50 model_photo_shoot.sdf

and the following msg should now be only printed once:

[Msg] Loading plugin [gz-rendering-ogre2]

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.

[ahcorde]

these test are failing

  	202 - UNIT_RenderEngineManager_TEST_ogre_gl3plus (SEGFAULT)
  	204 - UNIT_RenderEngineManager_TEST_ogre2_gl3plus (SEGFAULT)
  	206 - UNIT_RenderingIface_TEST_ogre_gl3plus (SEGFAULT)
  	208 - UNIT_RenderingIface_TEST_ogre2_gl3plus (SEGFAULT)

[iche033]

these test are failing

Should be fixed by e297225

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c62f6e9) 75.54% compared to head (e297225) 75.54%.

Additional details and impacted files

@@                Coverage Diff                @@
##           gz-rendering8     #961      +/-   ##
=================================================
- Coverage          75.54%   75.54%   -0.01%     
=================================================
  Files                177      177              
  Lines              16888    16891       +3     
=================================================
+ Hits               12758    12760       +2     
- Misses              4130     4131       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.