Fix export alignment (#7916)

Ardour's playback is aligned to master-out:
 "When the playback clock reads 01:00:00:00, the sample corresponding
  to 01:00:00:00 is audible at the speaker(s)"

When exporting, and grabbing data from output ports, the signal
is offset by the master-bus physical playback latency. This was
compensated for, but lead to initial silence in the exported file.

New approach is to start capturing export data during pre-roll,
at the time when playback is written to the output buffers.

To also shaves off a common offset to make this work with
realtime export. Effectively this emulates processing with
disconnected master-output port, while still keeping any
latency of effects on the master-bus itself.

Last but not least: jack updates latencies when freewheeling,
(setting HW latency to zero). The callback arrives asynchronously
some time after enabling freewheeling, but after Export
Ports have been configured. Those callbacks are ignored.

1 files changed, 1 insertions, 1 deletions

diff --git a/libs/ardour/session.cc b/libs/ardour/session.cc
index 74c0fbb22e..92d62274f7 100644
--- a/libs/ardour/session.cc
+++ b/libs/ardour/session.cc
@@ -6533,7 +6533,7 @@ Session::update_latency (bool playback)
 	if (inital_connect_or_deletion_in_progress () || _adding_routes_in_progress || _route_deletion_in_progress) {
 		return;
 	}
-	if (!_engine.running()) {
+	if (!_engine.running() || _exporting) {
 		return;
 	}