Guardians of the Galaxy star Vin Diesel will step into a superhero role for the second time when Bloodshot debuts on March 13. (Or third, if you consider Xander Cage a personal hero, as everyone should.) Unlike the usual box office giants of the comic book genre, the title character hails not from Marvel or DC, but the pages of Valiant Comics. It's understandable if your immediate reaction to a Bloodshot movie isn't "whoo!" but "who?", and we've got you covered. Here's everything you need to know about this pale-skinned badass with nanites in his …