THe captured pixmap should always have devicePixelRatio one, it only
needs to be changed when it is actually drawn in the preview widget.

Also fixes a regression from e4c2e564
("Replace KScreen by QScreen for current window grab"), now the current
screen grab mode will again capture the full screen on HiDPI screens.

Tested with: QT_SCALE_FACTOR=2 src/spectacle

REVIEW: 129189