When we open the connection, we get the wl_output object,
but we return before all the information such as monitor
geometry has arrived, which causes us to misinform early
users of this information. Do a roundtrip here that causes
us to wait until the information is complete. Do the same
for seats, just in case.
https://bugzilla.gnome.org/show_bug.cgi?id=747471
b2e19866ea