added -pix_fmt yuv420p to ffmpeg args if codec is libx264
added -pix_fmt yuv420p to ffmpeg args if codec is libx264

benroeder/moviepy,LunarLanding/moviepy,DevinGeo/moviepy,TheNathanBlack/moviepy,bertyhell/moviepy,Zulko/moviepy,ssteo/moviepy,carlodri/moviepy,kerstin/moviepy,Vicaris/ModPro,misterbisson/moviepy,mcanthony/moviepy,DevinGeo/moviepy,deniszgonjanin/moviepy