Fix using lightmap and emissive map together

[iche033]

🦟 Bug fix

Summary

To be used by the Ionic demo world.

Currently lightmap is applied through the emissive map slot. This means you can't use emissive map together with lightmap (only one of them can be applied). This was done to workaround an unwanted red hightlight issue found in gz-rendering6 + ogre2.2. This issue seems to be fixed now (as of gz-rendering9 + ogre 2.3). So I restored back to the original way of applying lightmaps through the detail map slot and I don't see the red highlights. Here's a screenshot of the lightmap sdf world with this change:

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)
• 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.