Fixes#1356 correctly.
This was previously fixed by #1360 which just assigned a random
placement ID. I asked the Kitty maintainer about this and this is not
the correct logic and the spec has been updated.
When a placement ID of zero is sent, we allow multiple placements but
use an internal, non-addressable placement ID. The issue with my
previous approach was someone with knowledge of internals (or luck)
could technically address the placement. This isn't allowed.