[FIX] Fix metalness and roughness handling in UsdzExporter

[willeastcott]

Fixes incorrect material appearance when exporting to USDZ format. Materials were appearing overly shiny/metallic in USD viewers.

Bug Fixes

• Gloss to roughness conversion: USD uses roughness (0=smooth, 1=rough) while PlayCanvas uses gloss (0=rough, 1=smooth). The exporter now correctly inverts the value: roughness = glossInvert ? gloss : 1 - gloss, matching the behavior in GltfExporter.

• Metalness for specular workflow: Materials using the specular workflow (useMetalness: false) were incorrectly exporting their metalness property (often defaulting to 1). Now exports metallic: 0 for specular workflow materials to prevent them appearing as reflective metals.

Example

For a material with shininess: 25 (gloss=0.25) and useMetalness: false:

• Before: metallic=1, roughness=0.25 (shiny metal)
• After: metallic=0, roughness=0.75 (matte non-metal)

Checklist

• I have read the contributing guidelines
• My code follows the project's coding standards
• This PR focuses on a single change

[Copilot]

Pull request overview

This PR fixes incorrect material appearance when exporting PlayCanvas materials to USDZ format. Materials using the specular workflow were incorrectly appearing as shiny metals in USD viewers due to two issues: (1) gloss values were not being inverted to roughness, and (2) specular workflow materials were exporting with their metalness property (which defaults to 1) instead of 0.

Key changes:

• Gloss-to-roughness conversion now matches GltfExporter pattern: roughness = glossInvert ? gloss : 1 - gloss
• Metalness is set to 0 for specular workflow materials (useMetalness: false) to prevent incorrect metallic appearance

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[mvaligursky]

perfect