Toy Story may have ended its three-film arc about as perfectly as anyone could ask for, but Pixar has been churning out short follow-up films for a while now to ensure that we get to keep up with the ongoing adventures of all of our pals. Now, they’ve put together “Partysaurus Rex,” a charming short that puts the neurotic Rex in the spotlight. It’s terrific, and it makes a good case for Pixar to keep putting these together for infinity and beyond …