Use ffmpeg to make kazam recorded mp4 available for Android devices

kazam is an easy to use screen record tool for Linux. User can install it via apt install kazam on Debian based distribution. After recording, kazam supports to save as mp4 files, which are 4:4:4 by default. To make it available for typical player on other platform, we can covert it to 4:2:0:

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ffmpeg -i ${original.mp4} -vf format=yuv420p ${new.mp4}