Don't invalidate textures when the scene graph is stopped.

The texture often represents the last reference to the last buffer of
a destroyed surface and invalidating releases that reference which
shortcuts any attempt to keep the buffer alive with QWaylandUnmapLock.

Change-Id: I4641a87a32f8639c3ffb6c3b5dafd21c77bbb2c1
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>