We had been doing it backwards as far back as e3966882e8 which meant
that we were translating by `fbo_offset / resource_scale` stage units
instead of just `fbo_offset`.
Because `fbo_offset` is in stage units already, it's not scaled and so
needs to be applied after the unscaling from texels to stage units.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1053>