Make an MPEG-4 version of the demo video

Apparently, Safari doesn't support h.264-encoded video with a frame size
larger than 640x480. Learned about it from this Stack Overflow answer:
https://stackoverflow.com/questions/20347352/html5-video-tag-not-working-in-safari-iphone-and-ipad/31247386#31247386

According to Apple's documentation:

> The following compression standards are supported:
>
> * H.264 Baseline Profile Level 3.0 video, up to 640 x 480 at 30 fps.
>   Note that B frames are not supported in the Baseline profile.
> * MPEG-4 Part 2 video (Simple Profile)
> * AAC-LC audio, up to 48 kHz

(https://developer.apple.com/library/archive/documentation/AppleApplications/Reference/SafariWebContent/CreatingVideoforSafarioniPhone/CreatingVideoforSafarioniPhone.html#//apple_ref/doc/uid/TP40006514-SW9)

Seems a bit odd to me, but I don't know video, so sure, why not?

Since I want to keep my atypical video dimensions, encode it in MPEG-4
following this guide:
https://trac.ffmpeg.org/wiki/Encode/MPEG-4

Unfortunately, this gives us a 12 Mb video file (compared to the h.264's
~3 Mb). Reducing the quality just didn't produce a decent-looking
output.

Deciding to keep the h.264 version, just in case it's supported
somewhere, because it's the version with the smallest file size.

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