Code review comment for lp:~vanvugt/mir/no-SurfaceBufferAccess

It does seem strange that ms::Surface has:
    std::shared_ptr<graphics::Buffer> snapshot_buffer() const;
and
void ms::BasicSurface::with_most_recent_buffer_do(std::function<void(mg::Buffer&)> const& exec)

with_most_recent_buffer_do() seems like it avoids ownership questions a bit better than handing out a shared_ptr, so I'd favor removing snapshot_buffer() from ms::Surface (snapshot_buffer should be private or removed)