qt/covergenerator: Restore QImage::scaled implementation

QImage::scaled provides a better quality compared to QImageReader::setScaledSize. Except for svg(s).

This implementation that was removed in a prior commit should stay relevant in the future.

Attached gif shows the difference, light gray background is before the patch, white is after (look at the building at the bottom center):

More samples, the blurry ones are with QImageReader::setScaledSize: