Pointer events are not mapped to correct output by nested Mir

/0/ Have a two display setup

/1/ Start a session compositor: bin/mir_demo_server with config:
    window-manager=system-compositor
    arw-file=
    file=/tmp/mir_socket
    vt=2
    display-config=sidebyside

/2/ Start a nested compositor: bin/mir_demo_server with config:
    window-manager=canonical
    file=/run/user/1000/mir_socket
    host=/tmp/mir_socket
    display-config=sidebyside

/3/ Start a client: mir/mir_demo_client_multiwin

Surfaces can be moved on the first screen [Alt+drag] by clicking on the corresponding part of the second screen.

(It also works correctly on the first screen - the problem is that the co-ordinates are not transformed from surface local to presentation space.)