This fixes several callsites that were passing samplepos_t to get a TempoMetric,
some of them somewhat significant (e.g. VST plugins that want tempo information).
Bad API design on my part, apologies.
This commit combines libs/ and gtk2_ardour because the new private status
of the ::metric_at() call would be a blocking point for git bisect
* Use an atomic reference count since the freeze-thread
can call use_playlist.
* Remove explicit argument to construct unused playlist
because playlists are unused by default. This also
lead to use-count becoming negative (or rather UINT32_MAX)
This fixes a random crash with stop-and-forget capture.
When aborting capture, the disk-writer can emit
midi_write_source->drop_references ()
in the butler thread, which leads to a direct call to
Session::remove_source.
This can happen before or after Region::source_deleted
is called which is initiated from the same signal.
Furthermore it was possible that Region::source deleted
was called concurrently from the UI thread via SourceRemoved
for whole file regions, which lead to memory corruption.
TransportMasterManager::destroy () destroys any remaining
TransportMasters which in turn unregister their ports.
However the PortEngine was already destroyed.
When starting RangePlay while the transport is already rolling
the transport is now stopped (and de-clicked) before locating.
This should not clear the RangeStop event when it is caused
by a RangePlay request.
In rare cases DiskWriter::run() may call finish_capture()
concurrently with the butler thread from transport_stopped_wallclock,
this can lead to memory corruption (CaptureInfo).
Using a Mutex here is not great, but it is not usually contended
and better than crashing at rec-stop.
We should probably change DiskWrWiter::_was_recording into an
atomic bool, and use CAS to prevent concurrent calls.
The GUI was able to free memory (MidiTracer::disconnect),
while the tracer is concurrently still in use in rt-context.
This lead to memory corruption in MIDI::Parser::scanner.
