Fix to autoresume from paused for live streams

If the Live media player is paused and the current position (playhead) gets close to dropping out of the playable window for the stream, the player automatically resumes. This fix adds a "cushion" of 8 seconds between the playhead and the start of the playable window, meaning that live players will autoresume 8 seconds earlier than they would have previously.

This is to tackle problems particularly seen in MPEG DASH streams, where the autoresume would occur just after the playhead had dropped out of the playable window, causing an error.