FfmpegVideo: add const_cast for earlier versions of ffmpeg.
FfmpegVideo: add const_cast for earlier versions of ffmpeg.

stevenlovegrove/Pangolin,renzodenardi/Pangolin,arpg/Pangolin,wxdzju/Pangolin,wxdzju/Pangolin,stevenlovegrove/Pangolin,pinglin/Pangolin,arpg/Pangolin,visigoth/Pangolin,mp3guy/Pangolin,tschmidt23/Pangolin,tschmidt23/Pangolin,mp3guy/Pangolin,visigoth/Pangolin,pinglin/Pangolin,randi120/Pangolin,renzodenardi/Pangolin,tschmidt23/Pangolin,randi120/Pangolin,mp3guy/Pangolin,stevenlovegrove/Pangolin