delete Image::_preMulti, Deprecate Image::isPremultipliedAlpha(), change Image::hasPremultipliedAlpha() meaning.
Deprecate Texture2D::PVRImagesHavePremultipliedAlpha(), move Texture2D::PVRImagesHavePremultipliedAlpha() to Image.
Change Image::PVRImagesHavePremultipliedAlpha() to Image::setPVRImagesHavePremultipliedAlpha()
Merge pull request #7594 from boyu0/image_premulti_alpha_refactor

Image premultiplied alpha refactor