Align windowShown flag with qml renderer state

Previously, we set windowShown to true once the window is active,
this is not enough for some tests as the initial rendering may not
be finished yet and will give the wrong result for some tests which
require reading back pixels.

Change-Id: Idd67329d207aaf1734a795b40a5bcc40093cf6b8
Reviewed-by: Michael Brasser <michael.brasser@nokia.com>