The pointer tracked internally by Wayland can be queried via
wl_proxy_get_listener, so we don't need our own bookkeeping.
This also changes the callback table's element type to a fixed-size uint64_t,
which makes it work for 32-bit guests.
This annotation can be used for data types that can't be repacked
automatically even with custom repack annotations. With ptr_passthrough,
the types are wrapped in guest_layout and passed to the host like that.
