bug#42607: guix.gnu.org/videos don't play on iOS

[Leo Famulari]

On Thu, Jul 30, 2020 at 01:08:48AM +0200, Tobias Geerinckx-Rice via Bug reports 
for GNU Guix wrote:
> nginx can easily sniff iOS/Macintosh user agents and serve them an inferior
> format to match, such as h264 with MP3 audio, which can both be encoded
> using free software in Guix.

Here is where the videos are created:

https://git.savannah.gnu.org/cgit/guix/videos.git/tree/Makefile

------
$(NUMBER).clino.$(VIDEO).webm: \
         $(VIDEO)/$(LOCALE_LANG)/durations.txt        \
         $(VIDEO)/$(LOCALE_LANG)/audios/all.mp3 
                ffmpeg -y -f concat -vsync cfr \
                -i $(VIDEO)/$(LOCALE_LANG)/durations.txt \
                -i $(VIDEO)/$(LOCALE_LANG)/audios/all.mp3 \
                -c:a libopus -b:a 192k \
                -c:v libvpx-vp9 -crf 31 -b:v 0 -pix_fmt yuv420p \
                -af apad -shortest \
                -vf fps=25 -threads 4 $(VIDEO)/$(LOCALE_LANG)/videos/$@;
                make clean_noCli
------

So, we'd need to add a target that did "-c:a copy -c:v libx264" and add
some logic to create two videos instead of one. I didn't look into it
closely yet to see if that will be simple or not.

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