ffmpeg demuxer: fast channel change
Credits: FernetMenta
ffmpeg demuxer: faster channel switching: import changes from @fetzerch
ffmpeg demuxer: faster channel switching: partial import of changes from @davilla for Pivos
Mainly documentation and better error handling

Source:
https://github.com/Pivosgroup/xbmc/commit/4abd884392526a5771b774fabe496618b1286b8a
https://github.com/Pivosgroup/xbmc/commit/b11bfc930742ac5005e22b2ab6c91527a38b3eb6