DIDN'T GET TO SEE THE ULTIMATE POP diva on tour? HBO gives you another chance with Barbra Streisand: The Concert (Sun., Aug. 21,9 p.m. ET), taped in Las Vegas and Anaheim, Calif. By running numerous clips on the Jumbotron screen, Babs manages to weave her film career into the show. (On tour she actually duetted with herself in Yentl on "Papa Can You Hear Me?") Accompanied by a 64-piece orchestra conducted by Marvin Hamlisch, she performs in sumptuous outfits she designed and had Donna Karan seamstresses sew up. In between songs she zings out "ad libs" carefully scripted by comedian Elayne Boosler. Obviously, Streisand spared no expense for this extravaganza. Of course, when you're charging $350 a ticket, why skimp?