Calling Session::set_play_loop repeatedly (e.g. LLL) should toggle in and out
of loop play. Previously transport needed to be stopped before loop play could
be started for a second or subsequent time. This uses the loop_changing boolean
to flag that Session::non_realtime_stop should not unset the loop.
Also, Session::non_realtime_stop must reset loop_changing to false after use so
it does not affect the next transport action.
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