fix QWidget::grab(QRect(x, y, -1, -1))

fix QWidget::grab(QRect(x, y, -1, -1))

Grabbing a widget with a position != (0,0) and an invalid size returned
a pixmap of the widget's size. As we're grabbing just a part of the
widget, the pixmap must be smaller by the amount of x, respective y.

Autotest: tst_QWidget::grab()

Change-Id: I3d5103e6e7c042cc6112038ace236e3f69060bda
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>