Otherwise we could fail to validate the new configuration depending on
which output we're initializing mirror mode from.
gnome_rr_config_applicable() creates a temporary array of outputs for
validation and, in clone mode, all of them are assigned the same
geometry but, since it lacks context about which output the user is
currently on, it just uses the first one's geometry for all of
them. If this first output isn't the one the user is changing in the
UI then we never called _set_geometry() on it with a clone mode
geometry and thus the validation would fail because the geometry used
is that output's current mode which might not match the clone mode's.
https://bugzilla.gnome.org/show_bug.cgi?id=743816
511265e574