Add KHR_materials_sheen export support

[willeastcott]

This PR adds export support for the KHR_materials_sheen glTF extension to the GltfExporter.

Changes

GltfExporter (src/extras/exporters/gltf-exporter.js)

• Export KHR_materials_sheen extension when material.useSheen is true:
  • sheenColorFactor from material.sheen (linear converted, if not black)
  • sheenRoughnessFactor from material.sheenGloss (if not 0)
  • sheenColorTexture from material.sheenMap
  • sheenRoughnessTexture from material.sheenGlossMap
• Added sheenMap and sheenGlossMap to texture semantics

glTF Extension Reference

The KHR_materials_sheen extension adds a sheen layer to simulate the appearance of cloth and fabric materials. The sheen BRDF models the microfiber structure of textiles.

Example Output

{
  "extensions": {
    "KHR_materials_sheen": {
      "sheenColorFactor": [0.8, 0.4, 0.2],
      "sheenRoughnessFactor": 0.5
    }
  }
}

Notes

• glTF's default sheenColorFactor is [0, 0, 0] (black/no sheen), so we export whenever sheen color is non-black
• The parser sets sheenGlossInvert = true, so sheenGloss directly maps to glTF's sheenRoughnessFactor

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 adds export support for the KHR_materials_sheen glTF extension, enabling the exporter to output sheen material properties that simulate cloth and fabric appearances.

Key Changes:

• Added sheen color and roughness factor export with appropriate default handling
• Added sheen texture export support (color and roughness maps)
• Registered sheen texture semantics for proper texture collection

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