Read the loop count from AVIS files.

[Zaggy1024]

The loop count is calculated as the number of times the animation must play to fill the trak duration based on the actual animation duration stored in the elst box.

I'm unsure if it will handle edit lists with multiple edits correctly, since it'll only take the first duration it finds, from my understanding of the elst parsing function, but I'm sure it's very unlikely to see more than one edit in practice.

[Zaggy1024]

For reference, this comment is useful for the specifics of how this should work:

https://github.com/AOMediaCodec/av1-avif/issues/73#issuecomment-605335922

[Zaggy1024]

It'd be good to add a test for the case where the loop count is not an integer and clarify the expected handling around that at the mp4parse C API level.

Done! Lemme know if there's anything else, otherwise I think it should be good to go.

[kinetiknz]

Thanks!