widgets forced to be native rather than alien

All widgets that are children of a scroll area (and that often mean a majority of widgets in an application), are native rather than alien as in 4.5.

This line added in qwidget_win.cpp 4.6.0 cause this (in function QWidgetPrivate::winSetupGestures)

winid = q->winId(); // enforces the native winid on the viewport

if removed, we are back to a "normal" situation concerning alien-ness and get better painting performances.