By THÉRÈSE MARGOLIS     The Thyssen-Bornemisza National Museum in Madrid is offering a first-ever exhibit on classic Spanish art and the extraordinary designs of the late Basque fashion designer Cristóbal Balenciaga, whose square coats and undulating bubble skirts revolutionized the international fashion world in the mid-1940s. Referred to as “the master of us all” by Christian Dior and idolized by Coco