ActiveStorage: Allow the parameters sent to ffmpeg to be configurable

[brendon]

Summary

The parameters sent to ffmpeg for generating a video preview image are now configurable under config.active_storage.video_preview_arguments.

This PR also better documents a previous commit from Jonathan Hefner (@jonathanhefner) that changes the default video image preview to use scene detection to generate a better preview.

Other Information

I don't think any additional tests are required since preview generation should be tested under both configurations in the CI suite.

Concerns: #39096

[brendon]

Looks like the failure was a transient failure to connect to S3.

[jonathanhefner]

Thank you for working on this, @brendon!

[pixeltrix]

Looks good but we usually have some configuration tests in railties to ensure that the defaults for upgrading apps and new apps are set correctly - can we add those please.

[brendon]

Ok, all the changes are made. Apologies for all the commits. I have to admit I don't know how to fix that. The guidance here didn't seem to work for me.

[brendon]

Managed to squish everything into one commit. Hopefully the tests are green and this is good to go :)

[pixeltrix]

@brendon thanks! 👍🏻

[brendon]

You're welcome @pixeltrix :) I'll fix up the back-port now :)

[crawler]

@pixeltrix, @zzak Sorry for the disturbance. Maybe, something like this will make sense for the Blob#default_variant_transformations ? For example auto_orient: true and/or gravity: 'Center' as default options can work well for all variants in some projects.

[pixeltrix]

@crawler actually I think a better feature would be to have named transformations, e.g.

config.active_storage.variations = {
  thumbnail: { resize_to_limit: [100, 100] }
}

avatar.variant(:thumbnail).processed.url

This seems fairly easy to implement by extending ActiveStorage::Variation.wrap to map a symbol to a key in the config hash.

Having said that I'm not opposed to also adding default transformations as long as it allows you to override the default file format as well to something like JPEG.

[crawler]

Or maybe allow to set defaults, through some combination with #39135

[pixeltrix]

LOL, didn't know about #39135 🤦🏻

[crawler]

I also just found this 😅