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: #4561 (closed)