Cocos2d-x v4 Inconsistent output from RenderTexture between OpenGL and Metal

[rh101]

• cocos2d-x version: 4.0
• devices test on: Android, Windows, Mac OS X 10.15, iOS8/10/11/12

Steps to Reproduce:

1. Generate a new project as you normally would, like this: cocos new gamename -p com.example.gamename -l cpp -d .

2. Change the code related to displaying the HelloWorld.png in HelloWorldScene::init() to this:

   
   auto sprite = Sprite::create("HelloWorld.png");
   sprite->setFlippedY(true);
   sprite->setPosition(sprite->getContentSize() / 2);
   auto* renderTexture = RenderTexture::create(sprite->getContentSize().width, sprite->getContentSize().height, PixelFormat::RGBA8888);
   renderTexture->beginWithClear(0, 0, 0, 0);
   sprite->visit();
   renderTexture->end();

auto* newSprite = Sprite::createWithTexture(renderTexture->getSprite()->getTexture());

// position the sprite on the center of the screen newSprite->setPosition(Vec2(visibleSize.width/2 + origin.x, visibleSize.height/2 + origin.y));

// add the sprite as a child to this layer this->addChild(newSprite, 0);

On Android/Win32 (OpenGL), this is the output:
![image](https://user-images.githubusercontent.com/8603230/99973956-6d107f80-2df4-11eb-9825-1d8af17dae4b.png)

On Mac OS X/iOS (Metal) this is the output:
![image](https://user-images.githubusercontent.com/8603230/99973873-59fdaf80-2df4-11eb-9d7a-cc76e838547d.png)

They should both be the same.  

[rh101]

@slackmoehrle As requested, adding you to this so you're notified about it. I'm not too familiar with the backend renderers, so I don't know the cause of this issue.

[slackmoehrle]

@minggo can someone take a look at this at this?