Fixes #3303. Makes Thickness a record struct

[tig]

Fixes

• Fixes Thickness should not be a reference type #3303

Proposed Changes/Todos

• class -> record struct
• Fix anything that breaks
• Use Vector4 under the covers per @dodexahedron's suggestion

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
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• I conducted basic QA to assure all features are working

[BDisp]

LGTM