Clean out embedded subs with the video buffer pruning

Created by: robertbryer

Fix for #3054 (closed) - Subs use up all memory

When the embedded subs are added there's no method to clear them out again. If you run a stream like the one on 3054 you can have a lot of subs in the text track, and if on Edge, there's a corresponding htmlElement for each one.

This PR watches for buffer cleared events and clears out any embedded subs between those times to keep the list small.