glTF Exporter: Fix spec-gloss conversion

[alexchuber]

From https://forum.babylonjs.com/t/broken-textures-after-glb-export/60756.

Changes:

• Add epsilon check to prevent division by 0
• Use new Color3 for maxBaseColor to avoid modifying the reference value Black
  • Make all reference values deeply immutable to prevent this in future
• Nit: remove dead code nearby

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