UI: Add VOD track support in advanced output

Adds a VOD track option (specific to Twitch) that allows a user to
specify which audio track to use for their Twitch VODs, which uses a
separate encoder to encode the track. This allows users the ability to
choose what audio goes on their VOD, separately from the live stream.

UI/window-basic-settings.cpp

@@ -3386,6 +3386,13 @@ void OBSBasicSettings::SaveOutputSettings()
 	SaveEdit(ui->advOutTrack5Name, "AdvOut", "Track5Name");
 	SaveEdit(ui->advOutTrack6Name, "AdvOut", "Track6Name");
 
+	if (vodTrackCheckbox) {
+		SaveCheckBox(vodTrackCheckbox, "AdvOut", "VodTrackEnabled");
+		SaveTrackIndex(main->Config(), "AdvOut", "VodTrackIndex",
+			       vodTrack[0], vodTrack[1], vodTrack[2],
+			       vodTrack[3], vodTrack[4], vodTrack[5]);
+	}
+
 	SaveCheckBox(ui->advReplayBuf, "AdvOut", "RecRB");
 	SaveSpinBox(ui->advRBSecMax, "AdvOut", "RecRBTime");
 	SaveSpinBox(ui->advRBMegsMax, "AdvOut", "RecRBSize");