We cannot be sure that the toplevel widget has the correct GL format,
and we do not want to force depth and stencil buffers on surfaces that
do not need them. Therefore, we have to create an offscreen surface for
the FBO.

Change-Id: I7dfc3a6f84bf79125f3ab811a204972e95e245a3
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>