Thumbnail helper doesn't handle 'Choice' resources correctly

[jbaiter]

When attempting to load the thumbnail for the Manifest at https://iiif.io/api/cookbook/recipe/0033-choice/manifest.json, getBestThumbnailAtSize is unable to find one:
https://codesandbox.io/s/vault-helpers-forked-lwux3g

I assume it's due to the fact that the Manifest's sole Canvas has an Annotation with a Choice body that has two different images.

Looking through the code, it seems to me that getBestThumbnailAtSize is missing a level of recursion. When it encounters an Annotation, it assumes that the first body item is an Image that can be passed to imageServiceLoader.getThumbnailFromResource: https://github.com/IIIF-Commons/vault-helpers/blob/8a14efa1fdaf2e31f87a926e515efb7bf3631bc3/src/thumbnail.ts#L72-L88

I think it should instead check the type of the body item and recurse if it's a Choice, so it will hit the branch at https://github.com/IIIF-Commons/vault-helpers/blob/8a14efa1fdaf2e31f87a926e515efb7bf3631bc3/src/thumbnail.ts#L113-L117, which should do the right thing.

[stephenwf]

Ah I see the problem, yes so maybe if it created a list of the content resources first, then it could pick from those. I think the general problem is that we need to check all possibilities before being able to choose the best one for a thumbnail.

There are a load of shortcuts, but I had hoped that this thumbnail helper would be a definitive way to always find a thumbnail if one existed. Will have a look over the weekend