La Barbecue pitmaster John Lewis and his new brisket pit. (Photo ©Chris Wilkins/Texas BBQ Posse) In Texas barbecue, competitive secrets are valuable. And a photograph, apparently, is worth much, much more than a few lousy words in a blog. No pictures. No pictures, La Barbecue pitmaster John Lewis kept repeating as he gave us a tour of his new pit during our recent tour of the best joints in Austin. Lewis claims he has built, with the help of his father, the world’s first offset convection cooker that operates without any mechanical assists, such as fans or blowers. Offset refers…