update to support ffmpeg 4.4? about libde265-ffmpeg

[MartinPulec]

If interested, I've updated the wrapper to current (master) version of FFmpeg, see here.

The main difference is that it cannot be compiled outside FFmpeg anymore - I think that this is no longer possible (easily) because the API for codecs has become internal in FFmpeg (before that it was public). On the other hand, the modified version doesn't replace the default HEVC decoder (which remains default), the libde265 can be opened, eg. via avcodec_find_decoder_by_name("libde265").

Otherwise it is more or less the same, I've done only 2 changes for me to work that I was not completely sure about (marked by @todo in code).