USDZExporter: Log a warning when matrixWorld has a negative scale.

examples/js/exporters/USDZExporter.js

@@ -141,6 +141,13 @@
 
 		const name = 'Object_' + object.id;
 		const transform = buildMatrix( object.matrixWorld );
+
+		if ( object.matrixWorld.determinant() < 0 ) {
+
+			console.warn( 'THREE.USDZExporter: USDZ does not support negative scales', object );
+
+		}
+
 		return `def Xform "${name}" (
     prepend references = @./geometries/Geometry_${geometry.id}.usd@</Geometry>
 )

examples/jsm/exporters/USDZExporter.js

@@ -155,6 +155,12 @@ function buildXform( object, geometry, material ) {
 	const name = 'Object_' + object.id;
 	const transform = buildMatrix( object.matrixWorld );
 
+	if ( object.matrixWorld.determinant() < 0 ) {
+
+		console.warn( 'THREE.USDZExporter: USDZ does not support negative scales', object );
+
+	}
+
 	return `def Xform "${ name }" (
     prepend references = @./geometries/Geometry_${ geometry.id }.usd@</Geometry>
 )