Merge: lib/sdl: workaround to avoid constructor callback bug

This was first a temporary fix for #1941 to repair Nit games on the deskop. But it is also a nice cleanup, it brings to Nit the instantiation and the implicit cast to a more general and nullable type. The casts themselves were not very pretty using FFI callbacks.

Pull-Request: #1960
Reviewed-by: Jean Privat <jean@pryen.org>