libobs: Pair encoders only when output actually starts

Normally, paired encoders are unpaired when they stop.  However, if the
pairing occurs before the encoders actually start, and the encoders
never actually end up starting, they are never unpaired, and that
pairing stays with them until the next time an output is started up
again.  That in turn can cause an output that uses one of the encoders
but not the other to not function correctly, and neither properly
"start" nor stop because the data is queued continually in the
interleaved packet array.

For example, let's say there are two outputs, two video encoders, and
one audio encoder.  This can be reproduced by using advanced output mode
and making the two outputs use separate video encoders while sharing
track 1's audio encoder.  If you start up the stream output first and it
fails to fully connect for whatever reason (bad server, bad stream key,
etc), then you start up the recording output, the recording output will
appear to be running, but will not stop when you hit "stop recording".
It will stay perpetually on "stopping recording" and will get stuck that
way.  This is because when the streaming output started, the streaming
output would initially pair video encoder A with audio encoder A before
the encoders actually fully started up (as the encoders do not fully
start up until a connection is successfully made), and when the
recording output starts up after that disconnection, audio encoder A
will wait for video encoder A rather than video encoder B because that
pairing was never actually cleared.

So, instead of pairing encoders when the output starts, wait until the
encoders themselves are being started and then pair the encoders at that
point in time.  This ensures that the encoders start up and will clear
their pairing when no longer in use.

libobs/obs-output.c

@@ -1756,9 +1756,6 @@ bool obs_output_initialize_encoders(obs_output_t *output, uint32_t flags)
 	if (has_audio && !initialize_audio_encoders(output, num_mixes))
 		return false;
 
-	if (has_video && has_audio)
-		pair_encoders(output, num_mixes);
-
 	return true;
 }
 
@@ -1785,6 +1782,7 @@ static bool begin_delayed_capture(obs_output_t *output)
 bool obs_output_begin_data_capture(obs_output_t *output, uint32_t flags)
 {
 	bool encoded, has_video, has_audio, has_service;
+	size_t num_mixes;
 
 	if (!obs_output_valid(output, "obs_output_begin_data_capture"))
 		return false;
@@ -1801,6 +1799,10 @@ bool obs_output_begin_data_capture(obs_output_t *output, uint32_t flags)
 				has_service))
 		return false;
 
+	num_mixes = num_audio_mixes(output);
+	if (has_video && has_audio)
+		pair_encoders(output, num_mixes);
+
 	os_atomic_set_bool(&output->data_active, true);
 	hook_data_capture(output, encoded, has_video, has_audio);