[packman] ffmpeg nvenc support

[Jimmy Berry]

I was asked about nvenc encoding support for obs-studio by
Mir_Poweredge in IRC. Since I do not have an nvidia test machine I
have never concerned myself with the support, but I decided to look
into it.

It looks like a new package with:
- nvEncodeAPI.h header files from the NVENC SDK

Then ffmpeg updating to include as build dep and --enable-nvenc added to config.

See https://trac.ffmpeg.org/wiki/HWAccelIntro#NVENC.

Does anyone have thoughts on legality of packaging the header file
from SDK or has any of this been discussed before?

--
Jimmy