Fix loading lightmaps from gltf / glb meshes

[iche033]

🦟 Bug fix

Summary

Fix loading gltf / glb mesh files with lightmaps. Needed by the Ionic demo world.

Currently the lightmap is ignored if there's a combined roughness + metalness texture. This PR fixes the problem by loading the lightmap (ambient occlusion data) from the roughness + metalness texture.

Tested with a few gltf sample models (water bottle, barramundi fish, damaged helmet)

Here are screenshots of the BarramundiFish before (left) and after (right) the change. I removed the albedo map to better visualize the effect of the lightmap:

Checklist

• [x] 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.

[iche033]

Some tests in UNIT_AssimpLoader_TEST on hombrew are failing. I noticed that they are failing in main as well and could be due to a new AssimpLoader homebrew bottle. I ticketed an issue for it: https://github.com/gazebosim/gz-common/issues/633

[iche033]

Fixed homebrew CI in https://github.com/gazebosim/gz-common/pull/634