HLS-19: Re-Asses Suitability of FFMPEGs HLS functionality

When HLS-5 came in, I took a cursory look at ffmpeg's HLS format to see whether it was suitable.

At the time, it was a fairly new feature and wasn't available in the version of ffmpeg available in Ubuntu repos. Time has passed though, so that's probably no longer the case.

Need to look at building a version of the script which uses the HLS format instead of segment_list to see whether switching will impact any existing or planned features.

The primary benefit over the existing implementation is that it will prevent TS Discontinuity errors between the segments (as segment_list resets the counter to 0 at each segment - IMO that's incorrect behaviour, but it's been the case for quite a while).