So Joss Whedon, TV cult hero and creator of the Buffyverse, has spent most of the last five years engaged in immensely lucrative struggle with a Marvel Comics mashup franchise. Toward what end, I am genuinely not sure. I have tried to take the question seriously: I went back and watched “The Avengers” again, and now I have seen its sequel, “Avengers: Age of Ultron,” also written and directed by Whedon.