Shai Gilgeous-Alexander stepped back, eluding Spurs forward LaMarcus Aldridge, and swished a 3-pointer. The Thunder guard took not one step, but two. It was his own version of the double-step-back jumper that James Harden has cleverly patented. Gilgeous-Alexander is still adding tricks to his game in his second season, and he revealed a new one Thursday night in the Thunder’s 109-103 win …