kcms/nightcolor: Fix click position for manual location
To get the click point in local coordinates of the image, including its scaling transform, we can use the position
property of the event point directly.
See https://doc.qt.io/qt-6/qml-qtquick-handlerpoint.html#position-prop
Tentative bugfix as I cannot reproduce it, but it doesn't make it worse for me with or without screen global scaling.
IF this actually fixed the bug, I will cherry-pick it along !3871 (merged)
BUG: 480892
FIXED-IN: 6.0