Code review comment for lp://qastaging/~mir-team/mir/animated-cursors

>> 91 === added file 'include/server/mir/frontend/buffer_stream.h'
>> Not sure this should be in the public api... but not sure the mf::Surface should be either.

I'm not sure off the top of my head about actual downstream usage but at least Session depends on frontend::Surface being public.

>> Also though, we have a mc::BufferStream, could the new interface be unified with mc::BufferStream?

Maybe though I didn't want to shove the observer methods on mc::BufferStream and wanted to follow the convention of defining interfaces in the namespace they are used (as much as I could)