Set proper metadata in transcode task output MP4

[victorges]

The output task is currently creating a video file that has no major_brand whatsoever in the MP4 container.
It also misses a lot of other metadata in the file, and browsers can't even play it back (they just prompt for
a download instead).

This is also a problem because the only way of detecting .mp4 vs .mov files is by checking the major_brand
of the ffprobe output.

Here is an example of such ffprobe output
not including a major brand, which was run on an asset transcoded by our own task. This is the problematic part:

      "tags": {
        **"major_brand": "",**
        "minor_version": "",
        "compatible_brands": "",
        "creation_time": "2020-01-10T19:37:32.000000Z"
      }

This kind of blocks #17 since we can't start depending on a field which we generate broken values ourselves.

[pglowacky]

@victorges Does this work relate to any ongoing product priorities? Or is this tech debt?

[victorges]

@pglowacky I think this is a medium severity bug. We, as a video-only platform, are generating malformed video files. So this seems like a pretty bad experience for users, also affecting their perception of the quality of our service.

This also blocks us from doing any logic regarding MOV files (#17), which are not supported on browsers. This means users can end up minting NFTs with video files that won't really playback anywhere but on Apple's Safari (although this is less of a problem now that we'll mint NFTs with the embedded player).

[pglowacky]

@victorges Thanks for the context, really helpful. Do have any sense of how frequently this bug is being encountered by users?

[victorges]

Every transcoded video file has this bug. So it happens 100% of the time that a user processes a video with our API.

[pglowacky]

Got it! This seems very high priority then -- if the output MP4 file can't be played in a player, that seems like it would break a users experience in a big way.

[pglowacky]

Moving to backlog because @victorges and I don't think this is a blocker for launch because we believe that most customers are going to use HLS playback for VOD assets rather than use the MP4, which users already have.