Composer Jacob Cooper’s opera, “Timberbrit” — “as in Justin Timberlake and Britney Spears” — is “a tragic tale that imagines Spears’ last concert, in the final hours of her life. … Cooper began work on the opera by experimenting with a technique called time-stretching. Using digital audio software, he slowed down Spears’ songs — and suddenly the light pop tunes seemed hauntingly tragic.”