fix: use jpg as a fallback format

[blt-r]

closes #15242

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@sveltejs/enhanced-img	Minor

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[benmccann]

• this would break transparency. if you have an avif or webp with transparency you'd lose that converting to jpg
• if the original image is a png or gif image, the intention was that you'd serve the original image as a fallback

[blt-r]

We can check metadata.hasAlpha? The only downside I can see to that, is that we'll always need to await metadata() and now we only do that when imgWidth is not passed. It is passed when user sets manually width prop, which is rare anyway. Even if it isn't, I don't think awaiting the metadata is much of a performance hit

[benmccann]

The only downside I can see to that, is that we'll always need to await metadata() and now we only do that when imgWidth is not passed.

We could skip doing that if the extension is .jpg or .jpeg, which it will be quite frequently. In that case, there's no need to check the metadata because we know the image cannot be animated or transparent and so we will need to stick to the jpg format

[blt-r]

We could skip doing that if the extension is .jpg or .jpeg

I don't think that's necessary, I've just looked at the source code and vite-imagetools caches the metadata object and later uses it itself to clamp the sizes to the intrinsic size of the image, unless allowUpscale is passed. So if we don't read the metadata it will be read later anyways.

[benmccann]

later uses it itself to clamp the sizes to the intrinsic size of the image, unless allowUpscale is passed. So if we don't read the metadata it will be read later anyways.

It looks like that's only true if w or h are manually specified

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-15243-svelte.vercel.app/

[blt-r]

w is always manually specified, in the defaultDirectives