Prevent Scene3D 💥 if another scene is already loaded

[chapulina]

🦟 Bug fix

• Part of Cleanup GUI plugin menu gz-sim#1254

Summary

Current issue:

1. Start ign gazebo
2. From the top-right menu, select Scene 3D
3. 💥

This PR prevents a crash and prints a message explaining to the user what happened.

The message will be printed if there's any other plugin already loading a render engine, so it's compatible with both GzScene3D and Scene3D.

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.

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[codecov]

Codecov Report

Merging #347 (9153833) into ign-gui3 (9af7abf) will decrease coverage by 0.05%.
The diff coverage is 64.28%.

@@             Coverage Diff              @@
##           ign-gui3     #347      +/-   ##
============================================
- Coverage     66.83%   66.77%   -0.06%     
============================================
  Files            25       25              
  Lines          2964     2992      +28     
============================================
+ Hits           1981     1998      +17     
- Misses          983      994      +11     

Impacted Files	Coverage Δ
src/plugins/scene3d/Scene3D.cc	43.06% <64.28%> (+0.67%)	⬆️

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[ahcorde]

Changes in these two files are unrelated:

• src/plugins/scene3d/Scene3D.qml
• test/integration/scene3d.cc

[chapulina]

Changes in these two files are unrelated:
src/plugins/scene3d/Scene3D.qml

This is necessary to display a message to the user about why the plugin failed loading.

test/integration/scene3d.cc

At first I thought that this PR had made the tests less precise, so I relaxed the tolerances. I don't understand how that could be the case though, I think that test may be flaky even without that PR. I could break that into another PR if it's clearer, but since I have the approval I'll go ahead and merge now.

[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-01-24-citadel-edifice-fortress/1241/1

[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-01-citadel-edifice-fortress/1313/1