[FFmpeg-user] merge quad layout in one run

[Roger Pack]

> then I actually found out I don't need it :)
>
> ffmpeg -i cam1.mp4 -i cam2.mp4 -i cam3.mp4 -i cam4.mp4 -filter_complex '[0]setpts=PTS+5/TB,drawbox=10:20:200:60:red at 0.5,pad=1280*2:720[upperleft]; [upperleft][1]overlay=1280:0[mergeupper]; [2]pad=1280*2:720[lowerleft]; [lowerleft][3]overlay=1280:0[mergelower]; [mergeupper]pad=1280*2:720*2[mergeupperout];[mergeupperout][mergelower]overlay=0:720' -ar 16000 -ab 64000 -y out-merged.mp4
>
> but with copy it made it clearer how it works.

There may be a way to use just one overlay command, but I'm not sure of it.
-r