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ACTORS AS DIRECTORS: Two movie stars make their debuts as cablefilm directors with a pair of powerful dramas premiering thisweekend. Forest Whitaker, the burly character actor who earned criticalpraise for his work in "The Crying Game," "Good Morning, Vietnam,""The Color of Money" and "Bird" (as jazz saxophonist Charlie Parker),reveals the forces behind an urban tragedy in "Strapped." As thesocially concerned filmmaker responsible for HBO's (STAR) (STAR)(STAR) 1/2 "Showcase" presentation, premiering at 7 p.m. tomorrow,Whitaker personalizes the scourge of firearms in America's cities.Bokeem Woodbine gives a breakthrough performance as a Brooklynteenager who does too many wrong …