Do not set shader constants for textures on non-OpenGL graphics API

[iche033]

🦟 Bug fix

Summary

This should fix the waves demo on macOS using ogre2 + metal. See #558 (comment)

I also had to backport the GraphicsAPI accessor function from #554

Waves demo now working again on macOS:

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)
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[codecov]

Codecov Report

Merging #574 (56c4300) into ign-rendering6 (e9516be) will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@                Coverage Diff                 @@
##           ign-rendering6     #574      +/-   ##
==================================================
- Coverage           54.38%   54.36%   -0.02%     
==================================================
  Files                 198      198              
  Lines               20152    20157       +5     
==================================================
  Hits                10959    10959              
- Misses               9193     9198       +5     

Impacted Files	Coverage Δ
...lude/ignition/rendering/ogre2/Ogre2RenderEngine.hh	100.00% <ø> (ø)
ogre2/src/Ogre2Material.cc	64.88% <0.00%> (-0.36%)	⬇️
ogre2/src/Ogre2RenderEngine.cc	77.92% <0.00%> (-0.36%)	⬇️

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