Code review comment for lp:~afrantzis/mir/consume-only-not-rendered-buffers

+ virtual std::chrono::steady_clock::time_point time_last_buffer_acquired() const = 0;

this makes Renderable leak the buffer stream abstraction even worse than it has before (I expound on this in my mp review: https://code.launchpad.net/~kdub/mir/produce-renderlist-for-particular-compositor/+merge/215052/comments/514979)