Troye Sivan is posing in full view of the street, delighting the small groups of teenagers who stop outside the photo studio to gawp. Some of them know who he is. The rest can tell he must be Someone. The South African-born Australian, 23, is as luminous as a cherub and has no qualms about giving it his best Rodin: foot perched on a starry plinth, trousers gaping around his slim frame. Sivan has been preparing to be looked at…