hls: adaptivedemux: Add support for EXT-X-START tag
Submitted by Hosang Lee
Link to original bug (#796434)
Description
Support EXT-X-START tag to start playing from a preferred point indicated in the playlist.
For EXT-X-START tag information refer to:
https://tools.ietf.org/html/draft-pantos-http-live-streaming-23#section-4.3.5.2