fail to build decord due to pointer convertion

[hubutui]

/home/hubutui/Projects/archlinuxcn-repo/archlinuxcn/python-decord/src/decord-0.6.0/src/video/video_reader.cc: In member function ‘virtual void decord::VideoReader::SetVideoStream(int)’:
/home/hubutui/Projects/archlinuxcn-repo/archlinuxcn/python-decord/src/decord-0.6.0/src/video/video_reader.cc:149:88: error: invalid conversion from ‘AVCodec**’ to ‘const AVCodec**’ [-fpermissive]
  149 |     int st_nb = av_find_best_stream(fmt_ctx_.get(), AVMEDIA_TYPE_VIDEO, stream_nb, -1, &dec, 0);
      |                                                                                        ^~~~
      |                                                                                        |
      |                                                                                        AVCodec**
In file included from /home/hubutui/Projects/archlinuxcn-repo/archlinuxcn/python-decord/src/decord-0.6.0/src/video/ffmpeg/ffmpeg_common.h:25,
                 from /home/hubutui/Projects/archlinuxcn-repo/archlinuxcn/python-decord/src/decord-0.6.0/src/video/threaded_decoder_interface.h:10,

quick fix, we could change AVCodec *dec; to const AVCodec *dec; in

decord/src/video/video_reader.cc

Line 148 in d2e5619

AVCodec *dec;

and if USE_CUDA=ON, then you also need to cast this pointer in

decord/src/video/video_reader.cc

Line 165 in d2e5619

ctx_.device_id, codecpar.get(), fmt_ctx_->iformat));

change fmt_ctx_->iformat to (AVInputFormat*)fmt_ctx_->iformat.

[gracemcgrath]

you might need to try the build again with ffmpeg 4 instead of 5. I referenced #203 and #186 to solve the same issues for macOS

[hubutui]

thanks, and my fix also works for ffmpeg 5.

[hubutui]

ref #257