Fixed #2852: The bug has been fixed that Scale9Sprite doesn't work correctly when it's texture loads asynchronous
Merge pull request #1189 from dingpinglv/Iss2852_RefactorMultResources

Fixed #2852: The bug has been fixed that Scale9Sprite doesn't work correctly when its texture loads asynchronous
merged develop branch from official repo
removed assignments to null in _beforeVisitScene and _afterVisitScene for the canvas case