Hardware acceleration as a flag for ffmpeg+opencv

[rsnk96]

Need to build ffmpeg with hardware acceleration (for Nvidia cards for starters)

[rajat2004]

As described in this answer:- https://stackoverflow.com/a/44518572 ?

Steps are given on Nvidia's page - https://developer.nvidia.com/ffmpeg

[rsnk96]

This documentation might be helpful @rajat2004
https://trac.ffmpeg.org/wiki/HWAccelIntro#NVENC

[rajat2004]

Can be closed after markus-perl/ffmpeg-build-script#34 is merged