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

*Needs Discussion*

I would expect a buffer stream to be something owned by surfaces, cursors, decorations. etc.

Given that I'm not convinced by the type hierarchy:

    Surface /IsA/ BufferStream Why now /HasA/?
    SurfacelessBufferStream /IsA/ BufferStream What is special about /not/ having a surface?

Are we going to have:

    Cursor /IsA/ BufferStream?
    Decoration /IsA/ BufferStream?

What about DecorationlessBufferStream and CursorlessBufferStream?

I don't feel I understand the intended roles well enough to suggest something better.