ffmpeg demuxer: faster channel change for PVR addons without internal demuxing
(such as MediaPortal, ArgusTV, MythTV, NextPVR)
Credits: FernetMenta, Davilla, Popcornmix
ffmpeg demuxer: refactoring for fast channel change
ffmpeg demuxer: reduce time for finding stream info
ffmoeg demuxer: make sure we start video with an i-frame