Previews don't respect height for portrait images using libvips #3376

Describe the bug

When requesting a preview image for a portrait image, it doesn't respect the given height, resulting in a much bigger returned image (~840kb instead of ~300kb for that image, that's almost 3x).

Note that this only affects libvips builds.

Steps to reproduce

1. Upload a portrait image (e.g. 4272 × 5712)
2. Request a 1000x1000 preview with processor: 'fit' (or just use the Web UI in the tiles view and inspect this request)

Expected behavior

The returned image's height is 1080 px, since this is the next biggest entry in the pre-defined list of resolutions.

Actual behavior

The returned image's height is 1920 px.