problem about "FFmpegDemuxer: no AVFormatContext provided" #287

Describe the bug
Hi @rarzumanyan , why did you remove "av_register_all( )" in this commit? I meet the question about "ValueError: FFmpegDemuxer: no AVFormatContext provided." which is related with #8 and #104, and I solved the problem when I add this line return to the code "FFmpegDemuxer.cpp".

To Reproduce
Steps to reproduce the behavior:

1. Compile and install
2. Run python SampleDecode.py 0 test.mp4 result.h264

Desktop (please complete the following information):

• OS: ubuntu 18.04
• Nvidia driver version: 440.64.00
• CUDA Version : 10.2
• Video Codec SDK Version: 9.1.23
• Python Version: 3.7.7
• FFmpeg Version: 4.4