ffmpeg demuxer: faster channel change for PVR addons ...

 without internal demuxing

(such as MediaPortal, ArgusTV, MythTV, NextPVR)

This pull request contains a Gotham port of the original work from @FernetMenta with a few changes from @Davilla
(https://github.com/FernetMenta/xbmc/commit/bf753ebec5f231bc7d85b6404c0841cf0307422d

It is a follow-up for #1757

FernetMenta asked me to port and test his work because he couldn't test it as a VDR user.