I could see the flashing purple party lights as I pulled up to Tropicasa, a vegan mini-mart and vintage-clothing shop in McAllen’s arts district. Empty, the space could serve as a garage for a Ford F-450 and not much else. But on this cold February night in the Rio Grande Valley, it was packed with several dozen locals dressed mostly in dark colors, bobbing their heads to a live deejay’s thunk-thunk-thunk as they celebrated the store’s fifth anniversary. No one looked much more than thirty years old. Tropicasa’s co-owner, Pedro Rodriguez, was just arriving with the night’s headliner, a Colombian deejay known as 2AT. Nodding greetings to their right and left, the two snaked their way through the crowd on the storefront porch packed with…