By Jenna Zucker, Steve Gorman and Moira Warburton BUFFALO, N.Y. (Reuters) -An 18-year-old white gunman shot 10 people to death and wounded three on Saturday at a grocery store in a Black neighborhood of upstate New York, before surrendering after what authorities called an act of “racially motivated violent extremism.” Authorities said the suspect, who was armed with an assault-style rifle and appeared to have acted alone, drove to Buffalo from his home several hours away to launch the afternoon attack that he broadcast in real time on social media platform Twitch, a live video service owned b…