Changing solo-state needs to be done in rt-context to atomically propagate solo/mute. set_control() queues a rt-event, later Session::rt_set_control() calls Session::update_route_solo_state() to propagate solo/mute.
Changing solo-state needs to be done in rt-context to atomically propagate solo/mute. set_control() queues a rt-event, later Session::rt_set_control() calls Session::update_route_solo_state() to propagate solo/mute.