workspace: Move fading minimized windows out of allocation

eee2ccac fixed window opacity being changed outside of transitions but
resulted in the final step of the transition, that would set the window
to fully opaque, being missed.

This change now moves the opacity change entirely out of allocation and
instead follows the adjustment changes directly, which still fixes the
original bug and ensures the opacity is also applied for the last step
in the transition.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4561
(cherry picked from commit b90a6d6b)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1982>