This effectively triggers a Session::request_overwrite_buffer
via Playlist::notify_contents_changed when the time-domain of a
Region changes.
When the time domain of a MIDI region changes, Region boundaries
and events positions change ever so slightly. The playlist has
to be rendered again to take this into account.
When MIDI regions with different time-domains are layered, notes
may not be resolved in correct order. Previously changing time-domains
to address this did not result in the playlist being reread.
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