Use an offscreen surface in VAO cleanup

Avoid failing the makeCurrent() on iOS that does not currently support
using a window with different contexts.

Change-Id: I2e10ad7e382161625a78518d02ad94edaff591ca
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@digia.com>
When looking up the window hierarchy, stop at foreign windows

If the window being activated is an embedded window, the parent window
will be a foreign window (usually not even belonging to the current
process); we shouldn't attempt to focus it.

Task-number: QTBUG-37984
Change-Id: I2ea03a86b30bbc43cde643e18e0e1d020e5f2c84
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>