Fix Obj group material propagation: fix #12542
Fix Obj group material propagation: fix #12542

RaananW/Babylon.js,RaananW/Babylon.js,sebavan/Babylon.js,BabylonJS/Babylon.js,sebavan/Babylon.js,BabylonJS/Babylon.js,BabylonJS/Babylon.js,RaananW/Babylon.js,sebavan/Babylon.js