ffmpeg -i src.mp4 -vcodec copy -an -f h264 src.h264
x264 --preset slow --tune stillimage --input-res 960x540 -o enc.h264 src.h264

ffplay enc.h264 # just shows colored noise

ffmpeg -i src.mp4 -acodec copy -vn src.aac
ffmpeg -i enc.h264 -s 960x540 -i src.aac -vcodec copy -acodec copy -bsf:a aac_adtstoasc enc.mp4

ffplay enc.mp4 # also just shows the same colored noise