Expose GraphicsAPI

[darksylinc]

🦟 Bug fix

There's no ticket assigned to this issue.

Summary

The current GraphicsAPI is only available at Ogre2RenderEngine level, which is problematic when we want to be aware of it for multiple means.

ign-gui needs this information so it can decide on how to interface the API with Qt.

In some cases initialization may need additional steps if different APIs are used; so this info is needed.

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.

[codecov]

Codecov Report

Merging #565 (6d8c27b) into main (1baa935) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #565      +/-   ##
==========================================
- Coverage   53.14%   53.12%   -0.02%     
==========================================
  Files         213      214       +1     
  Lines       21219    21225       +6     
==========================================
  Hits        11276    11276              
- Misses       9943     9949       +6     

Impacted Files	Coverage Δ
include/ignition/rendering/RenderEngine.hh	100.00% <ø> (ø)
...nclude/ignition/rendering/base/BaseRenderEngine.hh	0.00% <0.00%> (ø)
...nclude/ignition/rendering/ogre/OgreRenderEngine.hh	100.00% <ø> (ø)
ogre/src/OgreRenderEngine.cc	73.35% <0.00%> (-0.45%)	⬇️
...lude/ignition/rendering/ogre2/Ogre2RenderEngine.hh	100.00% <ø> (ø)
ogre2/src/Ogre2Material.cc	63.14% <0.00%> (-0.23%)	⬇️
ogre2/src/Ogre2RenderEngine.cc	78.36% <0.00%> (ø)

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[iche033]

looks good, just one minor comment