Add reason of track being added/removed in onremotetrack in janus.js

[marekpiechut]

Added a metadata object to janus.js onremotetrack callbacks with reason why track is being added/removed. Thanks to that users can decide if they want to handle the call or ignore it.

It can be used to fix blinking screen-sharing sessions as discussed here: https://groups.google.com/g/meetecho-janus/c/bZaQA4MqJcs
(user is being notified about track removal, when it's only a mute call)

After the change onremotetrack call will lok like this:

 onremotetrack: function(track, mid, added, metadata) {
            // A remote track (working PeerConnection!) with a specific mid has just been added or removed
            // You can query metadata to get some more information on why track was added or removed
            // metadata fields:
            //   - reason: 'created' | 'ended' | 'mute' | 'unmute'
        },

Change should be backwards compatible, as it's only adding a new parameter to callbacks that will be ignored by current client code.

I have tested demo pages with the change and it looks like everything is fine.
Also updated documentation.

[januscla]

Thanks for your contribution, @marekpiechut! Please make sure you sign our CLA, as it's a required step before we can merge this.

[lminiero]

Thanks! Not sure I like reason as a name for the property (maybe status?) but that's semantics. In principle I think the feature makes sense.

[lminiero]

As a side note, the demos should all have the signature of the callback updated, though. A ton of people won't look at janus.js or the documentation, and will assume the demos are the "verb". If they don't see the property there, they won't know it exists.

[marekpiechut]

Thanks for the review @lminiero. On Monday I'll go through the demos and update signature.
Sorry that it takes so long, but it's been a busy week.

[lminiero]

Sorry that it takes so long, but it's been a busy week.

No problem, take your time!

[marekpiechut]

I've added metadata to all demos onremotetrack callbacks. Most of them just display it in debug (had no good idea to actually showcase it).

For screen-sharing demo I've added code to ignore mute/unmute events with description why it might be useful (just like in my issue with blinking screen-sharing streams).

I've also found that in vp9svctest.js was using undeclared doSimulcast variable. This was crashing the demo. I've removed it and just disabled simulcast for this demo.

Let me know if something is still missing.

[lminiero]

Thanks! I added a few small notes.

[marekpiechut]

@lminiero I think I have fixed all the code style issues. I got too used to auto-formatters :)

[lminiero]

Thanks! Abroad for FOSDEM but I'll have a look as soon as I get back on Monday 💪

[lminiero]

Thanks, merging!