Allow grabbing windows rendered without showing for testing.

Allow grabbing windows rendered without showing for testing.

To make QQuickWindowPrivate::setRenderWithoutShowing more useful
for autotesting we can allow grabbing such windows. This is safe
if the platform window have been created and the window has a valid
size.

Change-Id: I8b9a2aaeb93935f662b75ef29651730b890441d5
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>