[Reg 5.15 -> 6.3] Bad image quality when scaling down QImage with SmoothTransformation

When scaling down a QImage with SmoothTransformation, some images get a bad quality - like grey noise over the image. In Qt5 it was working fine.

Please see the attached example project to reproduce. Running the example will show a down scaled QImage with bad quality.

The problem only occurs if you scale down an image with Qt::SmoothTransformation is set. Up scaling has no issues either.