QSurface: added Q_DECLARE_METATYPE

In Qt3D we need to be able to send a QSurface * through a QVariant which would
allow us to be as generic as possible.

Change-Id: I2ca2760f9b985c8dc1db822048fa41e13774d52c
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>