- System sound: on / off (red while on) turns what the computer plays on and off while
recording, as Mic does the microphone; its state is saved as system_audio in [video].
- modules/Screen_Record: the microphone's switch is now one for either source:
record_set_audio, record_audio_on and record_audio_failed take an Audio_Source_Kind, and the
audio thread opens and closes each as it is turned on and off.
- record-probe -system-toggle. With -sync-test, system sound on at 3 s and off at 7 s keeps only
the beeps at 3 and 5 s (8-12 ms from their flashes), and the track runs the whole recording.