Add a warning when using QOpenGLWidget as a native child

Just like it is done for QQuickWidget.

Task-number: QTBUG-41779
Change-Id: I1b27c2ed34ecb2520edf82843b675dbf6b0eab8e
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>
Print warning when unable to query physical screen size

This warning was removed when re-factoring code in:
328f2f9c35f3cc5e7049a060a608c3f72876484a

Change-Id: I5a9d7fbbf2b78e6e80a79478f4e9fb08ccaec431
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>