audioconvert: add monitor.passthrough option

Add a monitor.passthrough option. This will pass all latency information
directly between the port and its monitor ports.

This is interesting when the adapter (and audioconvert) is used with a
null-audio-sink that simply forwards the data to a real sink/souce. In
that case, we want the sink/source latency to be passed unmodified.

Set the monitor.passthrough on the pulseaudio null-sink because
a passthrough virtual sink is the most likely use case for this.

Add some monitor.passthrough default config where it makes sense.

Fixes #3888

src/daemon/minimal.conf.in

@@ -389,6 +389,7 @@ context.objects = [
     #        node.description = "Microphone"
     #        media.class      = "Audio/Source/Virtual"
     #        audio.position   = "FL,FR"
+    #        monitor.passthrough = true
     #        adapter.auto-port-config = {
     #            mode = dsp
     #            monitor = true

src/daemon/pipewire.conf.in

@@ -259,6 +259,7 @@ context.objects = [
     #        node.description = "Microphone"
     #        media.class      = "Audio/Source/Virtual"
     #        audio.position   = "FL,FR"
+    #        monitor.passthrough = true
     #    }
     #}
 

src/modules/module-protocol-pulse/modules/module-null-sink.c

@@ -196,6 +196,8 @@ static int module_null_sink_prepare(struct module * const module)
 
 	if (pw_properties_get(props, "monitor.channel-volumes") == NULL)
 		pw_properties_set(props, "monitor.channel-volumes", "true");
+	if (pw_properties_get(props, "monitor.passthrough") == NULL)
+		pw_properties_set(props, "monitor.passthrough", "true");
 
 	return 0;
 }