QScreen::grabWindow(): Scale the coordinates.

The coordinates need to be scaled before calling QPlatformScreen::grabWindow()
On return, set a devicePixelRatio on the pixmap.

Adapt the QWidget test to scale the grabbed pixmaps.

Fixes pixeltool displaying the wrong part of the screen when High DPI scaling
is in effect.

Task-number: QTBUG-46615
Change-Id: I12de7df0da669230cf0fae74f4a42d43f061d5ff
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>