Fix HLS muxing

[toots]

There was a subtile bug in HLS muxing:

• A id3 data stream is added to mpegts but metadata is sparse. However, ffmpeg still considers it as a stream to be interleaved.
• The default for muxing is to interleaved when there is more than one stream.
• The default logic above was broken.

This is tricky to fix so the changes here:

• Add a id3 packet at stream init. This unblocks initial interleaved write when needed.
• Fix the default interleaving logic

This shows good behavior with normal encoding params.

Some users will still need to forcefully set interleaved = false on the encoder.

The reason we're not doing this by default on hls streams is that this can also result in un-balanced A/V streams where too much audio is present etc.

tl;dr: HLS is hard and to make it work you need to properly understand your configuration. But for simple use-case like audio-only streams, this should behave just fine.

Fixes: #4398