Enable QComboBox animation on Windows and fixing related bugs

Enable QComboBox animation on Windows and fixing related bugs

QWidget:grab() has a default parameter (invalid rect).
QRect() is the null rect and widget->grab(QRect())
returns an empty pixmap.

QWidget:grab() has to be free of side effects and should not
affect any dirty flags.

Change-Id: I8ff558343c366769cd6c587e77f862668e8054f2
Task-number: QTBUG-25436
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@nokia.com>