Support skybox in wide angle cam view

[iche033]

🎉 New feature

Summary

Add support for setting sky (background material with a skybox cubemap) in wide angle camera view.

This is needed for harmonic demo but more of a feature than a bugfix so can come after release. No API/ABI breakages

Test it

See gazebosim/harmonic_demo#7

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• 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 #901 (f2da8bb) into gz-rendering8 (c1af6fd) will increase coverage by 0.02%.
The diff coverage is 85.41%.

❗ Current head f2da8bb differs from pull request most recent head 864981d. Consider uploading reports for the commit 864981d to get more accurate results

@@                Coverage Diff                @@
##           gz-rendering8     #901      +/-   ##
=================================================
+ Coverage          75.14%   75.17%   +0.02%     
=================================================
  Files                177      177              
  Lines              16832    16879      +47     
=================================================
+ Hits               12649    12689      +40     
- Misses              4183     4190       +7     

Files	Coverage Δ
ogre2/src/Ogre2Scene.cc	78.53% <83.33%> (+0.03%)	⬆️
ogre2/src/Ogre2WideAngleCamera.cc	85.41% <85.71%> (+0.16%)	⬆️

... and 1 file with indirect coverage changes

[azeey]

Let's hold off merging this till after the Harmonic release