Rangers have made a potentially title-deciding signing in Oscar Cortes with former Celtic ace Aiden McGeady full of praise for a man who has exploded out of the blocks under Philippe Clement at Ibrox. There was a moment, early on in Saturday’s Scottish Cup fifth round tie with Ayr United, which showed exactly what two of Rangers’ January additions are all about. First, Mohamed Diomande attempted to hook the ball in the net with an acrobatic volley and, after that failed, Oscar Cortes tried something similar, albeit also in vain. Clearly, while both efforts did not hit the net, Diomande and Cor…