[bugfix] - skip non-triangle mesh primitives in mesh join for navmesh recomputation

[aclegg3]

Motivation and Context

@mosra fixed a bug with non-triangle primitive support in #1888. This worked for physics, but was not being applied to navmesh recomputation resulting in artifacts for affected scenes. See comments for details.

How Has This Been Tested

Locally with FP scene 102816627.

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[mukulkhanna]

Thanks Alex! Yes, this seems to be working. Here's a before (without the above commit) and after (with the above commit) of a sample scene's topdown map: