Update MultiMaterial.js

Changed instanceof to cross-frame secure Array.isArray(array) 
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

src/materials/MultiMaterial.js

@@ -10,7 +10,7 @@ function MultiMaterial( materials ) {
 
 	this.type = 'MultiMaterial';
 
-	this.materials = materials instanceof Array ? materials : [];
+	this.materials = Array.isArray(materials) ? materials : [];
 
 	this.visible = true;