GLTFExporter: Better defaults for metallic and roughness factors. (#29692)

* GLTFExporter: Better defaults for metallic and roughness factors.

* GLTFExporter: Stick to glTF spec.

* GLTFExporter: Clean up.

examples/jsm/exporters/GLTFExporter.js

@@ -1524,8 +1524,7 @@ class GLTFWriter {
 
 		} else {
 
-			materialDef.pbrMetallicRoughness.metallicFactor = 0.5;
-			materialDef.pbrMetallicRoughness.roughnessFactor = 0.5;
+			materialDef.pbrMetallicRoughness.metallicFactor = 0; // default roughness is 1
 
 		}