Support delayed create of SourceBuffer

[fightZy]

Is your feature request related to a problem? Please describe.

I have an M3U8 playlist with only audio data for the first few seconds, and then both audio and video data are available. At this point, hls.js will report an error because the video sourceBuffer cannot be found. By checking the hls.js code, I learned that the sourceBuffer seems to only be created during initialization based on the track type owned by the first ts file;

Describe the solution you'd like

Can you support changing existing sourceBuffers based on actual changes in track types? At least for me, I need to adapt to the addition of track types