boxes: Actually check for rectangle containment

Fixes condition duplicated:

          /* If a contains b, just remove b */
          if (meta_rectangle_contains_rect (a, b))
            {
              delete_me = other;
            }
          /* If b contains a, just remove a */
          else if (meta_rectangle_contains_rect (a, b))
            {
              delete_me = compare;
            }
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00:09:29