Don't keep around old dirty region when entire window is invalidated

QPaintDeviceWindowPrivate::markWindowAsDirty() is used to signal that
the entire window needs repaint, for example when the window is being
resized. If multiple resize events come in before we have a chance to
redraw the window, we'll end up redrawing the window based on the
window's current size and exposed region, as we should, but we'll
still have a dirty region, because we didn't redraw the window at
any of its old (larger) sizes. Resetting the dirty region instead
of appending to it should be enough.

Change-Id: I37443cc1044779f847348a2303a6cebc1a8f3bcb
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
(cherry picked from commit 774095ed9aea96e3a9e56441079735cd726c072a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/kernel/qpaintdevicewindow_p.h

@@ -82,7 +82,7 @@ public:
     void markWindowAsDirty()
     {
         Q_Q(QPaintDeviceWindow);
-        dirtyRegion += QRect(QPoint(0, 0), q->size());
+        dirtyRegion = QRect(QPoint(0, 0), q->size());
     }
 
 private: