wayland/window: Don't lose precision in MetaWaylandWindowConfiguration

Commit 8bdd2aa7 would offset the window position by the difference
between the configured window size and the committed size from the
client to prevent the window from drifting while resizing.

This, however, did not take into account the actual geometry scale, so
when using any scale greater than 1, the window would rapidly drift away
due to that offset.

In order to solve this, we need to make sure we store away the pending
window configuration in the stage coordinate space, in order to not
loose precision. When we then calculate the offset given the result from
the client, it'll use the right scalars, while before, one scalar was in
surface coordinates, while the other in stage coordinates.

https://gitlab.gnome.org/GNOME/mutter/-/issues/1490
(cherry picked from commit eaa6efef)