MetaWindowActor: fix updates with multiple size changes in one frame

We must set size_changed even if we are frozen, as every window
size change makes the X server drop the pixmap, and we might lose
the information at the next thaw() if the window changes size
twice in one frame (so we would keep drawing with the old pixmap
until something else causes another resize)

Fix done together with Giovanni Campagna <gcampagn@redhat.com>