Fix lack of "layerschange" event when all streams in the producer die

[ibc]

The initial purpose of this PR was to JUST enable RTP inactivity check in simulcast streams (this was already done in PR #1061) and just if the video has more than 1 single stream (remember that if the producer video is VP8/H264 with a single stream/encoding with N temporal layers, it generates a SimulcastConsumer). And this has been done already. However I've found another issue (regression that happened no idea when).

So here the problem:

• Simulcast with > 1 streams.
• So RTP inactivity checks are enabled in all streams.
• All Producer streams die so RTP inactivity happens in all them.
• SimulcastConsumer::ProducerRtpStreamScore() is called in all them with score: 0.
• Such a method should trigger 'consumerlayerschange' with spatialLayer: null and temporalLayer: null, but it doesn't (pretty sure it did in the past).
• Why? Because SimulcastConsumer::ProducerRtpStreamScore() ends calling this->MayChangeLayers() which ends calling this->listener->OnConsumerNeedBitrateChange().
• Transport::OnConsumerNeedBitrateChange() ends calling this->DistributeAvailableOutgoingBitrate() which calls consumer->GetBitratePriority().
• SimulcastConsumer::GetBitratePriority() checks this->IsActive() and it returns false so Transport forgets about this consumer and won't call IncreaseLayer() etc, so the consumer won't have a chance to notify null layers.
• SimulcastConsumer::IsActive() returns false because it's check the score of all the streams in the producer and, if all 0, it returns false.
• So here the problem. Not sure how to resolve it. But I did: https://github.com/versatica/mediasoup/pull/1122/files#diff-24e17490eee048f39100d99aee1f3b0f842a73ebca6128162e3e838443c07b4bR1336

[jmillan]

SimulcastConsumer::GetBitratePriority() checks this->IsActive() and it returns false so Transport forgets about this consumer and won't call IncreaseLayer() etc, so the consumer won't have a chance to notify null layers.

How can it not be active at this point? MayChangeLayers() is only called if the Consumer is active.

[ibc]

Because all the streams of the producer now have score 0 so SimulcastConsumer::IsActive() returns false. And because I think we call MayChangeLayers() by just checking if parent Consumer::IsActive() which is true.

[jmillan]

I wonder if the following change in SimulcastConsumer::ProducerRtpStreamScore would be more clear and explicit:

diff --git a/worker/src/RTC/SimulcastConsumer.cpp b/worker/src/RTC/SimulcastConsumer.cpp
index 3527f3c0f..a5d4a44aa 100644
--- a/worker/src/RTC/SimulcastConsumer.cpp
+++ b/worker/src/RTC/SimulcastConsumer.cpp
@@ -324,9 +324,14 @@ namespace RTC

                if (RTC::Consumer::IsActive())
                {
+                       // All Producer streams are dead.
+                       if (!IsActive())
+                       {
+                               UpdateTargetLayers(-1, -1);
+                       }
+
                        // Just check target layers if the stream has died or reborned.
                        // clang-format off
-                       if (
+                       else if (
                                !this->externallyManagedBitrate ||
                                (score == 0u || previousScore == 0u)

EDIT: diff lines may not match with 'v3' since I was in a different branch.

[ibc]

@jmillan pushed here. Tested. It works. 331d42b