Only windows that are part of the application can be grabbed. This
excludes the system statusbar and other system overlays, as well
as windows of other applications. This is a limitation due to the
security model of iOS.
There exists APIs to grab a snapshot of the complete screen, but
these APIs return a view that can be used as a placeholder e.g.
during view transformations, and doesn't match our API that require
reading of pixels.
Task-number: QTBUG-49944
Change-Id: I8fd5b4c2777be1486f0ff22762d5e9b64c927e70
Reviewed-by: Jake Petroules <jake.petroules@qt.io>