fix: Model collision issues with more complex meshes

[Eideren]

PR Details

Collision shapes using models as input for both bullet and bepu did not merge the different meshes appropriately.
Additionally, bepu did not account for meshes with skeletons either.

Related Issue

fixes #3018

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 change requires a change to the documentation.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have built and run the editor to try this change out.