- Needs a mechanism to pass buffers to client.

      buffer = drm.create_buffer(); /* buffer with stuff in it */

      cache.upload(buffer, x, y, width, height, int hash)

      drm.buffer: id, name, stride etc /* event to announce cache buffer */

      cache.image: hash, buffer, x, y, stride /* event to announce
                                              * location in cache */

      cache.reject: hash   /* no upload for you! */

      cache.retire: buffer /* cache has stopped using buffer, please
                            * reupload whatever you had in that buffer */