Muxed tracks

[kixelated]

The current catalog assumes that each track can only contain a single codec (demuxed). We should probably support muxed tracks too, although I'm not sure the best way to do this.

[wilaw]

Could we just use the existing MSE convention of listing multiple codes separated by a comma? For example:

"codec":"avc1.4D4028, mp4a.40.2"