Support large QGraphicsWidgets on Mac.

Problems reproduced in the qgraphicsgridlayout test.

The calls to "widget->resize(widget->effectiveSizeHint(Qt::MaximumSize));" exposes several problems on the Mac:

• We are trying to allocate a bitmap with the same size as the widget somewhere.
• In QGraphicsWidget::paintWindowFrame, Calling QPainter::setClip with the mask returned by the SH_WindowFrame_Mask style hint causes QRegion to create a region with 170000 rects (see task 16481).
• QGraphicsWidget::paintWindowFrame tries to draw a rect of size (1.67772e+07x1.67772e+07), which crashes.