allow set alpha premultiplied value for texture

[drelaptop]

allow set premultiplied value manually, this will solve render texture grey border

issues like this:

• render a white label in white layer to render texture, but grey border appears
• set premultiplied value to true fixed it, after that render texture become to whole white.

[drelaptop]

this PR related with #16113 partly, but now fix saveToFile grey border issue. @PatriceJiang

[minggo]

premultiplied flag is used to set blend function, i don't think it is a good idea to set it directly in texture2d

[drelaptop]

or manually do like:

auto texture = renderTexture->getSprite()->getTexture();
auto sprite = Sprite::createWithTexture(texture);
sprite->setBlendFunc(BlendFunc::ALPHA_PREMULTIPLIED); //need to do this!

it's so internal, much hard to find it out.

[minggo]

Yep, the design of premultiplied is not good, but it is hard to change it now.

[minggo]

close this one as #19782 is merged