(performances are Thursday through Saturday) The Skirball Center for the Performing Arts has a single stage rather than three rings and its supply of cotton candy is scant. But that hasn't stopped it from hosting this new festival devoted to circus entertainments. Its stable of international clowns and acrobats includes the feminist aerialists Girls on Trapeze (from the United States), the physical comedians Parallel Exit (also from the United States) and the Australian acrobats Casus.