The compositor's output_created signal used to be sent in weston_output_init()
which the backend call before putting the output in the output_list.
This caused problems when creating a new view in a listener to that signal,
because weston_view_assign_output() doesn't yet know the new output exists.
To fix this add a new weston_composito_add_output() func which adds the
output in the list and later sends the signal, and make the backends call
that.