Update to ffmpeg 4.3
Updated a bit the script so next major upgrade should be a bit simpler, all in all it was quite simple tbh. I am still having a linking issue with vs2017 build and don't have time to check right now so I put it here so we can work move from here.
I basically cherry-picked everything from 4.1 and updated from there, so reviewing the last 3 patches sould be the only interesting thing to review.
Fixes #13 (closed)
Edited by Nirbheek Chauhan