Code review comment for lp:~alan-griffiths/mir/fix-1376324

Hmm, actually SurfaceSnapshot doesn't really need the BufferStream at all. The SurfaceSnapshot is created at the instance when we want to composite. We should instead just pass it a buffer, and not let it hold a reference to the BufferStream. In theory that should guarantee that BasicSurface holds a unique pointer (which we should enforce as unique_ptr) to BufferStream and thus it would become impossible for the BufferQueue to live longer than the surface it calls back into.

I think that would be a nicer (and smaller?) fix, if it works.