Fix item visibility check when building GI Vct (backport #1216)

[mergify]

🦟 Bug fix

Summary

Add better check when adding an item to the voxelizer when building GI

Previously, the objData.mOwner[k]->getVisible() check may fail and cause a crash if objData is not derived from an Ogre::Item. This PR makes sure that it's an Ogre::Item first before checking visibility.

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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This is an automatic backport of pull request #1216 done by [Mergify](https://mergify.com).