A gunman was shot dead by police in downtown Dallas yesterday morning after opening fire outside a federal building. Veteran photojournalist Tom Fox was on the block when the shooting started, and he managed to bravely capture a photo of the masked man staring down the sidewalk into his camera. The Dallas Morning News reports that Fox, a photographer with the paper for nearly 3 decades, was waiting to enter the Earle Cabell Federal Building to cover a charter school…