Create BG context and fake surface on GUI thread

This makes the example more messy and also makes it very hard
to avoid the initial black frame, but it is not compliant with
the limitations we experience on Windows.

Task-number: QTBUG-30077
Change-Id: If5a6da16f8fc1269cd20d2aa1190588025203d4f
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>