After a rocky first two episodes, HBO’s “The Comeback” — with Lisa Kudrow as a former sitcom star gamely hanging on in the business by taking a supporting role in a horrible new show AND starring in her own reality show about her new supporting role — has become an extraordinary work of televison comedy. Hilarious, painful, clever, with Kudrow giving what may turn out to be one of the great comic performances ever filmed as somebody who is really, really, really, REALLY trying to keep it all together despite life’s efforts to drive her insane.