Fix: add a r168 compat to MToonNodeMaterial

Starting from r168, `setupNormal` returns the normal node instead of assigning inside the `NodeMaterial.setupNormal`

See: mrdoob/three.js#29137

packages/three-vrm-materials-mtoon/src/nodes/MToonNodeMaterial.ts

@@ -252,7 +252,7 @@ export class MToonNodeMaterial extends THREE.NodeMaterial {
     parametricRim.assign(this._setupParametricRimNode());
   }
 
-  public setupNormal(builder: THREE.NodeBuilder): void {
+  public setupNormal(builder: THREE.NodeBuilder): THREE.ShaderNodeObject<THREE.Node> | null | undefined {
     // we must apply uv scroll to the normalMap
     // this.normalNode will be used in super.setupNormal() so we temporarily replace it
     const tempNormalNode = this.normalNode;
@@ -271,11 +271,25 @@ export class MToonNodeMaterial extends THREE.NodeMaterial {
       }
     }
 
-    // the ordinary normal setup
-    super.setupNormal(builder);
+    // COMPAT r168: `setupNormal` now returns the normal node
+    // instead of assigning inside the `super.setupNormal`
+    // See: https://github.com/mrdoob/three.js/pull/29137
+    const threeRevision = parseInt(THREE.REVISION, 10);
+    if (threeRevision >= 168) {
+      const ret = this.normalNode as THREE.ShaderNodeObject<THREE.Node> | null;
 
-    // revert the normalNode
-    this.normalNode = tempNormalNode;
+      // revert the normalNode
+      this.normalNode = tempNormalNode;
+
+      return ret;
+    } else {
+      // pre-r168
+      // the ordinary normal setup
+      super.setupNormal(builder);
+
+      // revert the normalNode
+      this.normalNode = tempNormalNode;
+    }
   }
 
   public setupLighting(builder: THREE.NodeBuilder): THREE.Node {