Prevent loading lightmaps if mesh is a glb file that has an occlusion-metallic-roughness texture

[iche033]

🦟 Bug fix

Summary

When loading this water bottle glb file, I manually saved the lightmap texture file (loaded by assimp) to examine what it looks like. It turns out the lightmap texture is just a copy of the occlusion-metallic-roughness texture, which is not correct. There are issues reported upstream (see inline comments in code).

Currently if an occlusion-metallic-roughness texture exists, we extract the roughness texture map from the g channel and the metalness map is in the b channel of this texture. I suspect that the right thing to do is to extract the r channel as a separate occlusion texture and use that as the lightmap (not quite the same but occlusion maps are not yet supported in gz-rendering). This needs to be investigated. For now, this PR prevents setting incorrect lightmap textures for glb meshes.

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)
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[codecov]

Codecov Report

Merging #538 (fcaf0d2) into gz-common5 (0e866d8) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head fcaf0d2 differs from pull request most recent head e02a4c7. Consider uploading reports for the commit e02a4c7 to get more accurate results

@@              Coverage Diff               @@
##           gz-common5     #538      +/-   ##
==============================================
- Coverage       83.65%   83.64%   -0.01%     
==============================================
  Files              90       90              
  Lines           10249    10243       -6     
==============================================
- Hits             8574     8568       -6     
  Misses           1675     1675              

Files	Coverage Δ
graphics/src/AssimpLoader.cc	88.97% <ø> (-0.25%)	⬇️

[iche033]

the homebrew warning should be fixed in #529