QWidget::grab: Add a default value for the QRect parameter.

QWidget::grab: Add a default value for the QRect parameter.

The documentation is misleading in that a default-constructed
rectangle that has size = 0 causes the entire widget to be
painted; this happens only if the one of the size values is
 < 0.

Add a default value such that size < 0 and the entire widget is
painted based on the rationale that a size = 0 might be the result
of some calculation and it would be unexpected to get a pixmap
containing the entire widget in that case.

Change-Id: I2a5fb38cd407b4b64348213101d8196b2c40ad9a
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>