package/ffmpeg: bump version to 2.6.1

Dump two Kodi-specific patches according to
https://github.com/xbmc/xbmc/pull/6636


and update two new patches to improve hevc handling with Kodi Isengard.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>