Get a transparent QPixmap back when grabbing transparent QWidgets with QPixmap::grabWidget

QPixmap::grabWidget always returns an opaque pixmap, even if the widgets grabbed are all transparent. QPixmap::grabWidget should provide an overload that allows the caller to specify the color with which the pixmap should be initialized, or initialize the pixmap with the background role of the widget, and respect value of autoFillBackground property of the widget grabbed.