Clean up internal resources on Destroy

[iche033]

🦟 Bug fix

Summary

Motivated by gazebosim/gz-sim#2084.

Went through and clean up resources and pointers when the objects destroyed. This is is to make sure that we don't hold on to any pointers after the engine is destroyed.

More info:
We have ogre pointers as member variables in different rendering classes. The destructors are called when they go out of scope. If the engine is destroyed before the destructors are called, the destructor may be calling function from pointers that are no longer valid. So this PR tries to clear and reset all pointers when objects are destroyed (before the engine is unloaded).

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 #886 (ecbbfef) into main (2286452) will increase coverage by 0.01%.
The diff coverage is 94.11%.

❗ Current head ecbbfef differs from pull request most recent head 874acb0. Consider uploading reports for the commit 874acb0 to get more accurate results

@@            Coverage Diff             @@
##             main     #886      +/-   ##
==========================================
+ Coverage   75.13%   75.15%   +0.01%     
==========================================
  Files         176      176              
  Lines       16835    16851      +16     
==========================================
+ Hits        12649    12664      +15     
- Misses       4186     4187       +1     

Files Changed	Coverage Δ
ogre2/src/Ogre2SelectionBuffer.cc	96.28% <87.50%> (-0.29%)	⬇️
ogre2/src/Ogre2LensFlarePass.cc	99.31% <100.00%> (+0.02%)	⬆️
ogre2/src/Ogre2Material.cc	76.07% <100.00%> (+0.06%)	⬆️
ogre2/src/Ogre2RayQuery.cc	89.90% <100.00%> (+0.09%)	⬆️