gdk/wayland: Avoid idempotent wl_subsurface.set_position calls
These may not result on wl_surface.frame callbacks, yet we do trigger a frame clock tick that would get stuck on the lack of such callback.
Closes: #844 (closed)
Given the mutter improvement still makes sense, I would like to move this patch forward. This is a new incarnation that fixes all the issues seen along the first try:
- Completion windows don't get stuck
- Video subsurface in totem is positioned correctly. Both fullscreen and not, and both on weston and mutter.