Flushing native child widgets in VG and GL window
surfaces caused performance downgrade because unnecessary
swapBuffers calls. On Symbian we must not support flushing
native child widgets in VG and GL window surfaces because
it causes GPU memory overhead and performance issues. Symbian
graphics architecture allows us to render native child widgets
to TLW EGL surface correctly in most of the cases.
Task-number: QTMOBILITY-1570
Reviewed-by: Samuel Rødal
(cherry picked from commit 6a92de7c89764848f7a85b1aa412a07bedc72b1a)
806873f9af