Prevent windows outside the gui thread in rendercontrol example

Prevent windows outside the gui thread in rendercontrol example

In the multithreaded case we attempted to create QWindows outside
the gui thread on platforms where QOffscreenSurface is backed by
the hidden QWindow in the absence of a better solution. This has
to be avoided. Therefore, pass a suitable surface from the gui
thread instead.

This will avoid "Attempting to create QWindow-based QOffscreenSurface
outside the gui thread." type of warnings when running on OS X with
--threaded.

Change-Id: Ie3ebeeeaa9e6bdf83e763e40213e2940fbfde667
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>