Fix backdrop image fetching to prioritize non-language images

[bradbeattie]

Context

#8946 accidentally introduced a side effect of all backdrop images preferring the explicit language of the library. The purpose of backdrop images should actually deprioritize language.

We could additionally modify OrderByLanguageDescending, but how this gets used with both manual image searches and during metadata refresh makes it a little slippery to refactor properly. The proposed solution here is fairly simple and gets us the behaviour we're looking for.

Changes

Image sorting prioritizes non-language images for backdrop metadata refreshes.

[crobibero]

I think a better change for this is to update DownloadMultiImages:

- foreach (var image in images.Where(i => i.Type == imageType))
+ foreach (var image in images.OrderBy(i => string.IsNullOrWhiteSpace(i.Language)).Where(i => i.Type == imageType))

[bradbeattie]

I think a better change for this is to update DownloadMultiImages:

- foreach (var image in images.Where(i => i.Type == imageType))
+ foreach (var image in images.OrderBy(i => string.IsNullOrWhiteSpace(i.Language)).Where(i => i.Type == imageType))

My concern with that change is that it binds DownloadMultiImages to specifically favour non-language images. True, it's only used for that purpose at the moment, but if imageType is an option, best to leave the sorting to calling function.

[crobibero]

Ok, I would still change the sort method to what I suggested

[bradbeattie]

Ah! So the intent behind listWithNoLangFirst is to say "I'll observe whatever ordering is defined in OrderByLanguageDescending, but otherwise prefer non-language images first". Because https://github.com/jellyfin/jellyfin/blob/master/MediaBrowser.Model/Extensions/EnumerableExtensions.cs#L19 might have complexity unknown to listWithNoLangFirst.

I'm unclear if C# ordering with .OrderBy(i => string.IsNullOrWhiteSpace(i.Language)) will preserve the ordering of elements in its list that are equal in the lambda evaluation, but otherwise unique.