wayland/plasmashell: Double buffer position
During a resize the client will provide a new position every frame, the new global position needs to be applied in sync with when a buffer is attached otherwise resizing can look glitchy.
This is most useful with Qt 6.9 where we have properly synced resizing. On Qt 6.8 this should not regress, but it won't fix anything useful.
It's a bit of a behavioural change, but it's internal API.