Add private data class to Image

[iche033]

🎉 New feature

Summary

Pimpl'ize Image class

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 #875 (4860af8) into main (9a76f1f) will decrease coverage by 0.01%.
Report is 2 commits behind head on main.
The diff coverage is 71.42%.

❗ Current head 4860af8 differs from pull request most recent head abea76a. Consider uploading reports for the commit abea76a to get more accurate results

@@            Coverage Diff             @@
##             main     #875      +/-   ##
==========================================
- Coverage   75.10%   75.09%   -0.01%     
==========================================
  Files         176      176              
  Lines       16853    16859       +6     
==========================================
+ Hits        12658    12661       +3     
- Misses       4195     4198       +3     

Files Changed	Coverage Δ
include/gz/rendering/Image.hh	50.00% <50.00%> (ø)
src/Image.cc	78.12% <72.22%> (-6.50%)	⬇️
src/RenderEngineManager.cc	76.89% <100.00%> (ø)