overlaying time/frame accurate

- extracting all frames from time 03:32 to 03:35 to get the "point of interest" timing:

$ ffmpeg -ss 00:03:32 -i in.mkv -vf fps=29.970628 -t 3 out_%d.png

the point of interest happens at frame number 50, then the timing is: 03:(33+(50 / 29.970628)) = 00:03:34.668

then overlaying the picture from 2 points of interest like this:

$ ffmpeg -i in.mkv -i in.png -filter_complex "[0:v]scale=768:-1[scaled];[scaled][1:v]overlay=enable='between(t,3*60+34.670,3*60+39.630)'" -t 220 test8.mkv