Fix flicker on image change
After transition, the source of frontImage got set to the same source as bufferImage and right after opacity switches. So the frontImage got displayed right after source change, even if not yet loaded.
Now the opacity change moved to frontImage's onStatusChanged event, so no flickering occurs. The faderAnimation.restart() moved to bufferImage's onStatusChanged event for the same reason, even if no flickering was visible.
The bufferImage now gets sourceSize.width and .height as well, since otherwise it is not "smoothly filtered when scaled or transformed" (https://doc.qt.io/qt-6/qml-qtquick-image.html#smooth-prop) and on opacity switch the quality of the image changes.