rendercontrol example: Add missing flushes

Otherwise we will show broken output with some drivers.
QQuickWidget does this already.

Change-Id: I5193595c4606d0497dbe85f6f8c670618ca366e9
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>
Update scenegraph docs regarding threaded render loop usage

Change-Id: I4f8aa9b209096bbcdabcb689007e4166279f8a4c
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>
Return a Heap::Object in Object::prototype()

Change-Id: Ice0265ae558ba14497421a5bbf25ee9db76adab5
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>