Don't try to grab Windows with invalid or a zero size
Don't try to grab Windows with invalid or a zero size

It will end up in LiveLock trying it. Because the Window will never
appear in the Rendering thread and setting the grabContent.

On the other end, grabWindow will never return because it waits for the
RenderingThread to set grabContent

Change-Id: Ia1c365acf033732b3e2e9d9d7dc8e73f9e8f4870
Reviewed-by: Gunnar Sletta <d0b71651f3efaa9fda200bbcc395b7b82a871fdd@digia.com>

qmlc/qtdeclarative,crobertd/qtdeclarative,corngood/qtdeclarative,corngood/qtdeclarative,jfaust/qtdeclarative,jfaust/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,crobertd/qtdeclarative,mgrunditz/qtdeclarative-2d,qmlc/qtdeclarative,crobertd/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,jfaust/qtdeclarative,matthewvogt/qtdeclarative,jfaust/qtdeclarative,crobertd/qtdeclarative,jfaust/qtdeclarative,corngood/qtdeclarative,matthewvogt/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,qmlc/qtdeclarative,corngood/qtdeclarative,crobertd/qtdeclarative,corngood/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,qmlc/qtdeclarative,corngood/qtdeclarative,mgrunditz/qtdeclarative-2d,matthewvogt/qtdeclarative,corngood/qtdeclarative,jfaust/qtdeclarative,qmlc/qtdeclarative,jfaust/qtdeclarative,crobertd/qtdeclarative,mgrunditz/qtdeclarative-2d,crobertd/qtdeclarative,matthewvogt/qtdeclarative