Fix Safari backwards seek bug

Created by: robertbryer

There's a bug in Safari on macOS, where appending a segment to the start of a buffered range causes a portion of the original start of the buffer to be removed.

To reproduce this bug: Seek to a point in the video, buffer for a few seconds. Seek back 15-20 seconds, play the video forward to the first seek point. The video stops for lack of buffer at the point first downloaded segment. Dash.js believes it has already downloaded this segment, so will fetch later ones instead.

This fixes it by watching the onSeeked event, and then removing any buffered ranges ahead of the currentTime(for macOS user agents only). There's no point just re-fetching and replacing the missing segment because the problem occurs again filling in that gap.