WebXRManager: support multiple views (#23972)

* WebXRManager: support WebXR multi-view

* WebXRManager: cleanup camera init

* WebXRManager: cleanup

src/renderers/webxr/WebXRManager.js

@@ -635,7 +635,16 @@ class WebXRManager extends EventDispatcher {
 
 					}
 
-					const camera = cameras[ i ];
+					let camera = cameras[ i ];
+
+					if ( camera === undefined ) {
+
+						camera = new PerspectiveCamera();
+						camera.layers.enable( i );
+						camera.viewport = new Vector4();
+						cameras[ i ] = camera;
+
+					}
 
 					camera.matrix.fromArray( view.transform.matrix );
 					camera.projectionMatrix.fromArray( view.projectionMatrix );