Expose QPlatformWindow::invalidateSurface as a virtual function.

This can be quite useful on some embedded systems to free up
graphics memory when windows are not used. QEglFSWindow already
implements the function.

Change-Id: I79b08efbd3c67d7be34df6a0e12dd184a92d48c5
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>