Code review comment for lp:~vanvugt/mir/remove-unused-session-param

OK, well since the session attribute isn't functionally required anywhere within the Mir code, I suggest a different approach: surface.set_session(s);

So you can query the surface session, but it's more clear that it's not a required parameter for surface construction, which it obviously isn't. That would reduce constructor-coupling and allow us to keep most of the simplifications proposed here.