vips_thumbnail: Prefer width over height or vice versa

I like to see a (optional) prefer option in vips_thumbnail which can be set to a new enum (let's call it VIPS_PREFER_WIDTH or VIPS_PREFER_HEIGHT for example).

Use case scenario:
To prevent reduction or enlargement in the horizontal axis, I'm setting the target resize width to VIPS_MAX_COORD (which is currently 10000000), when a user only have specified a fixed height. I'm doing vice versa for the target resize height (so setting it to VIPS_MAX_COORD when a fixed width is specified). This feels somewhat hackish.

Example which currently fails due to reduction in the vertical axis (the dimensions of the input JPEG-file are 1600x622):
https://gist.github.com/kleisauke/8e090b4f83679cd536cd3bc7aebe60a2

thumbnail width = 499
expected width = 500
thumbnail height = 194
expected height = 194

This is fixable if we change the script to this:
https://gist.github.com/kleisauke/a7df1817fa7ed8f01a627a7eccd342c4

thumbnail width = 500
expected width = 500
thumbnail height = 194
expected height = 194

When a prefer option is in vips_thumbnail we can do:
https://gist.github.com/kleisauke/1dc424dfdf1b2b65192be4189a4c6364

Start at the vips_thumbnail entry point and reproduce the linked JPEG example to understand the current dimension selection. Review the proposed prefer option and enum behavior in the linked examples; done means fixed-width and fixed-height requests choose the preferred axis without the current one-pixel reduction or enlargement.