Jacopo Tintoretto’s awkward “Nativity,” at Boston’s Museum of Fine Arts, has been hiding something. “In the Renaissance equivalent of a cut-and-paste job, it appears that Tintoretto changed his mind about the subject, cut the original canvas to rearrange the pieces he didn’t like, then – perhaps two decades later – painted over parts of the result to come up with an entirely new composition. The painting that is now a horizontal nativity was once a vertical crucifixion.”