Update MaterialLoader.js

Honor `allowOverride` in `MaterialLoader`.

src/loaders/MaterialLoader.js

@@ -203,6 +203,7 @@ class MaterialLoader extends Loader {
 		if ( json.alphaToCoverage !== undefined ) material.alphaToCoverage = json.alphaToCoverage;
 		if ( json.premultipliedAlpha !== undefined ) material.premultipliedAlpha = json.premultipliedAlpha;
 		if ( json.forceSinglePass !== undefined ) material.forceSinglePass = json.forceSinglePass;
+		if ( json.allowOverride !== undefined ) material.allowOverride = json.allowOverride;
 
 		if ( json.visible !== undefined ) material.visible = json.visible;