Visa problems have resulted in the cancellation of another artist’s visit to Tanglewood. Earlier this season it was tenor Gwyn Hughes Jones, who had been slated to perform in Act I of “Tosca.” Now, the composer and conductor Oliver Knussen has been forced to cancel his appearance at the Festival of Contemporary Music, where he was to conduct a keenly anticipated memorial program in honor of the composer Gunther Schuller, featuring premieres by Schuller and Charles Wuorinen, on Thursday evening.