Allow RenderTexture to save out non-premultiplied alpha images

[rh101]

• cocos2d-x version: 3.17.1
• devices test on: Win32
• developing environments
  • NDK version: N/A
  • Xcode version: N/A
  • VS version: VS2019

Steps to Reproduce:

The current implementation of RenderTexture has several issues.

Firstly, it does not set the premultiplied alpha flag on the image created by this method:

Image* RenderTexture::newImage(bool flipImage)
{
...
            image->initWithRawData(buffer, savedBufferWidth * savedBufferHeight * 4, savedBufferWidth, savedBufferHeight, 8);
        }
        else
        {
            image->initWithRawData(tempData, savedBufferWidth * savedBufferHeight * 4, savedBufferWidth, savedBufferHeight, 8);
        }
...
}

The correct code would be to change the initWithRawData calls to this:

            image->initWithRawData(buffer, savedBufferWidth * savedBufferHeight * 4, savedBufferWidth, savedBufferHeight, 8, true);
        }
        else
        {
            image->initWithRawData(tempData, savedBufferWidth * savedBufferHeight * 4, savedBufferWidth, savedBufferHeight, 8, true);

Secondly, there is no way to choose if you want to save the file as a non-premultiplied alpha PNG.

Reversing the PMA before saving the image is possible, and a new method can be added to do this, such as: RenderTexture::saveToFileAsNonPMA().

I'll create a pull request with the changes, along with modification of the tests to cater for this new method.

This fix may work for the following reported issues:
#18980
#18193