PHOTOS BY JAY LUSTIG Gangstagrass (from left, Dolio the Sleuth, R-SON the Voice of Reason, Brian Farrow, Rench and Dan Whitener) performs at SOPAC, Feb. 5. Energetic and forward-looking, Gangstagrass was a fitting headliner for the celebratory grand reopening concert of the South Orange Performing Arts Center, Feb. 5. There are other groups that combine elements of Americana music and hip-hop, but no one else does it the way this one does, piling head-spinning raps on top of fast-and-furious bluegrass instrumentation. There is, of course, an uplifting subtext to this kind of cross-cultural fus…