There’s a difference between a pop-culture touchstone and a pop-culture curiosity, and the original “Scooby-Doo” cartoon – an unremarkable series that’s gained notoriety only because we’ve sat through decades worth of reruns – qualifies as the latter. Even so, the endless miscalculations on the filmmakers’ parts have turned the movie version into more of a disaster than anyone could have imagined. With its creepy, computer-generated Scooby and its unnecessarily labyrinthine plot, this summer bust can’t even evoke feelings of nostalgia.