If you’ve got a smartphone, Facebook friends, and a pulse, you’ve probably been subjected to friends checking in on Foursquare, Gowalla, Yelp, and now Facebook Places. So you’re already sick of Foursquare’s mayors – and the last thing you want to hear about is Yet Another One of Those Things. But SCVNGR, a Cambridge-based startup that has Google’s backing and a mandate to build “the game layer on top of the world,” has been quietly outpacing its larger rivals in location gaming, and this week it begins a hometown blitz that could change that game for good.